My mom hides things constantly and then says they are stolen and that she is going to report it. I need ideas of what to say to her.

It is likely the most common complaint. When you think of it "our stuff" is a lifelong thing. Keeping our stuff, having enough stuff, not letting others get our stuff. So it just gets real bad when the mind fails and kicks into overdrive. You won't stop it. For my Partner, when his Mom began to fail and had a lovely woman caring for her daily it was always "Wilma took it". It was a constant litany. Wilma took my blender Wilma took my pearls Wilma took my purse. It went on so long that after she was gone it because a joke with us. Now that he is 80 and I am 78 whenever we mislay something we say to one another "Wilma took it". When we enter care we will likely STILL be saying it.

Its common behavior. My mom puts things in odd places. You never know what you will find.
I organize her things and she undoes it. I have removed all valuable and important things from her room.

If your Mom has been diagnosed as having dementia, she will not respond to anything you say about her issues with missing items, or gestures addressed to items that are missing.

Her actions are happening because her brain is losing the ability to make sense of what is happening around her. Your logical interpretations of what she is doing do not fit with what her brain is telling her from moment to moment.

Can you respond to her with a neutral comment to make her comfortable, rather than toward the act she talks about? Saying you’ll look into it is fine. Then change the subject to something else, perhaps family chit-chat or the weather or sports?
Anything that might draw her in.

Saying she can’t stay there much longer is very typical behavior for someone newly placed in residential care. Your response again needs to be open ended.

You're describing my mom and many others as this is very common. In my case she lives with me so I am that mystery "someone"! Let me save you some grief on the disappearing items, (I hope). She will always misplace and forget, it doesn't get better. We tried a safe. She lost the key, "someone" stole it. Finally we got her a safe deposit box and put her money and jewelry in it. Boundaries were set, someone would take her on their schedule not hers if she wanted access. We have some peace. But it's not a total solution because we're not going to have her non valuables such as her face cream in the safe deposit box. At the least, we know her true valuables are safe and won't disappear in the trash etc. Good luck on this so very frustrating issue.

Maybe she’s rite and she can’t stay there alone much longer just not for the exact reasons she thinks. But just an idea to try, if she isn’t frightened by this mystery intruder and it’s just a real nuisance from her perspective maybe, just maybe, you can turn her anger and frustration into more of a game or something. This entity stole half a can of ginger ale? They must have been thirsty, sure hope their tummy is good with carbonation! She keeps hiding her keys from this person, maybe it would be better to leave them out where the person can find them and steal them so they can be caught red handed. Everybody needs toilet paper now, poor person, it’s nice Mom can help them out. My mom sometimes has hallucinations about strangers being in her room, she doesn’t seem frightened by them at all so I started commenting on how nice it must have been to have a visitor, some company. I started asking if she spoke to the “girl” and when she hadn’t I suggested she try that next time. Now she seems very aware that she has hallucinations so even when it very real to her I can gently say I checked the camera and didn’t see anybody, she even asks me to first and she says I guess it must not have happened then and seems to rest assured that it was just all in her mind. It’s all about approach with Mom, not that you know what the right approach is going to be at any given time because it’s a moving target but still it helps me to know I can often control her reactions by the approach I take, then I can break down and cry or scream later.

Either way, there is likely no reasoning with your mom on this or convincing her it isn’t happening, the more you fight it the more she will likely dig in so try going with it and just changing the perspective, if you will. Good luck!

Sounds like she has anxiety. There are many med options.... something that won’t konk her out and calm her paranoia.

This may be a perfect opportunity to make the move to memory care or assisted living. Mom, it's so much safer there and they have others living there they protect from intruders too. They have the best security ever! In her mind she will be safe and sound?