They aren't just gonna dissapear... Logically it really does sound like someone is stealing things... It's not normal. Your SO should address this with his he!!ions.

Have you gone looking for the missing items and turned up nothing?
Usually the items are buried in the pit of a bedroom. IF you find the items take them back, and let your SO know that these items have no need to be in their bedrooms.

I had tons of kitchen stuff disappear...I eventually just stopped buying it.  If DH doesn't care, then I don't.  If we run out of bowls to use, then so be it.  I never had things disappear from a kitchen EVER in my life - funny it only started when DH and I lived together with SDs....

Note:  My Good, Good stuff (glasses, antique plates, wooden utensils, porcelain knives, etc.) are all stored in a cardboard box in the pantry.  No one else uses those b/c they don't think to look in there.

But only when the skids have been there.  Literally the lid to the trash can.  The roller for toilet paper, countless pieces of silverware and dinnerware.

In our case, it's that the skids are disrespectful.  They don't care about our stuff (or anything) because their mother hasn't taught them to care about anyone but themselves.

The summer right before Ss23 left for college, Dh and I took a short vacation.  That was the summer that Ss had a party at our house while we were gone.  Underage drinking and the whole bit.  Which he lied about it all.  I kept finding evidence of the party and one thing was that stuck out was that our can opener was missing.  I did find a big can of pineapple juice in the trash when we returned and it was opened with a can opener. 

I made Ss buy a new can opener with his own money even though he denied that he or his friends used it. Lies, lies, lies.

I then took inventory of my silverware and found that pieces were missing.  I think that happened over a longer period of time and he would just throw them away on accident.  At least I hope it was on accident.  Of course he would never in a million years admit to it.

With his ADD, he didn't pay attention to much detail.

Time for Nanny cams and a lock for your bedroom door.

I say an old fashion prison style shakedown of all the rooms is in order.  Let them clean up the mess after you are done.  Forget privacy for now.  They haven't earned it.  If its your stuff that is missing then you have every right to go looking for it and teach the lesson to the skids about your boundaries and how to live with you. When they complain and whine just tell them if they didn't like those consequences then keep their grubby paws off your stuff.  

towels.   When DH and I combined households, the only thing we had in abundance was bath towels.   DH didn’t have much else to bring.    Shortly after moving in I noticed we had fewer and fewer towels. Eventually I caught in it was ALL of The towels that DH brought that we’re missing.  Every single one.   

I mentioned it to DH but like all step kid isssues at that time, he ignored it.   I knew what happened, Oss had gym class so he would take a towel from our house every Tuesday evening, it would either end up left at school or at BMs house never to be seen again.   But yet when Oss stopped coming over, our towels stopped disappearing....go figure.

Oss didn’t take one of my towels.  Probably because he knew I would lose it if he did.   

Oss always was a jerk...I doubt much has changed since I saw him last.

It seems a lot of stepkids aren't taught that other people's belongings have value and are off limits. I taught this to my stepdaughter when she was really young. DH was totally on board. From 2 years old onwards, if she touched something that wasn't hers, she was reminded that it's not hers and handed something of her own. For example... Coffee table books. If she picked one up, we'd take it, hand her her own book, and remind her that she has her own things and we have our own things. It took about a year for her to really understand, but by 3 years old, she stopped touching things that weren't hers. 

Even now, at 7 years old, she doesn't touch when it's not hers. Even in the shower, she knows to use her own soap/shampoo and not touch anyone elses. It's quite easy. Whereas my SIL and her two kids (4 and 5) stayed with us for a month or so and they were completely different. I bought $100 worth of cute christmas decor from target and brought it home. She said "oh man I hate to tell you but that's all going to be broken fast if you put them out" and I was in complete shock. Why would MY items be broken? Excuse me. To her, it was just normal for her kids to take/play/break whatever was laying around. 

And it's true. Her kids touched and played with EVERY SINGLE ITEM I put out for decoration. Everytime I stopped them, she would get this annoyed attitude like "why put it out if my kids can't touch it?" it drove me insane. 

Oh yesss.  Our home was a favorite for missing items. Escalating to  vandalizing property both heirlooms and personal items AND exterior and interior of our home.  It went on for several years. Subtle at first then becoming more and more purposeful.

There were  blobs of mucus and feces "accidently" on walls near the end. Toilets accidently over flowing, personal expensive items missing, sums of money stolen....family heirlooms accidently broken. Kitchen items passed down 2 generations broken...family animals hurt requiring vet treatment.  1 of which died shortly thereafter.  My bio's were also hurt.

There came a point were I slept with bedroom doors locked. We were very scared after finding knives in skid drawer.

Of course some events could not be proven while others were clearly proven.

Here is what I can say about all of it....IT stopped immediately when dh said NOT anymore in our home.  That was years ago. Purely magic right?

I know there are others on who have had similar situations.  When dh and I put everything together in a story line it is re traumatizing to relive those years. We still look over our shoulder and flinch seeing certain makes of vehicles.

BM would park her vehicle in such a way when we walked out of our home we could see just the front end tip  of it.  I would imagine now a'days it would be considered stalking.

Family courts do not do enough to protect decent families who are good members of society from this. They continue to reward it with money.....in the form of cs. Bad behavior should never be rewarded in this manner.

There are several people i know who experienced the same things I wrote about. It's awful...all becaue overnights equal money. Kids taking over inside the home where bm for the most part can not reach. 

PS I am sorry you can not find your Nana's blanket. That is how it starts out...things that are important to you go missing or 'accidently' broken. Hide it all OP,,,hide it all. Go buy a large safe that is impossible to pick up. OR rent a very small storage unit IF you have a lot of things.

A lesson we learned too late is, 'There are no coincidences".

Some people tell us,  they will get their Karma....well I have yet to see it. Therefore I do not believe in Karma.

Yeah. I had missing stuff too. Silverware, dishes, random kitchen utensils, towels, blankets, spices (probably used for stupid youtube "challenges"), hair products, thousands in jewelery, of course money.

It was flippin rediculous and formerSO "just couldn't believe SS was stealing all that stuff". REALLY?! Because he has been caught red handed - at home AND at school. SMDH.

One of the many things I couldn't deal with.