94 yo mom who lives in assisted living loses things constantly then can’t remember where she “hid” it. Suggestions??

1 Help look for the object, a few minutes to acknowledge how they feel.
2 If you can't find it, give substitute, if you have one.
3 Redirect their attention to something else, like an activity or conversation.

She should probably move into Memory Care, which is a shrunken down world far smaller than Assisted Living. As my mother's dementia progressed, she could no longer function in the larger AL apartment with more gizmos like a microwave & fridge, cleaning supplies under the sink, otc meds in the bathroom, etc., so we had to move her next door into the Memory Care bldg. She still loses stuff, but they turn up quicker since she now has a suite which consists of a bedroom & a bathroom so the square footage is much smaller with less areas to 'lose' things in. There is no way to prevent them from losing things...........you just limit their stimuli and space, get them lots more help (the caregiver to resident ratio is MUCH better in MC than in AL) and that's about it. In MC, mom is supervised all the time in the activity room, and then checked on every 2 hours while in her suite. So, if/when she loses things or can't work the phone/remote, a CG is never far away.

Good luck!

If the item is not "important" like a piece or box of tissue, an (empty) purse, her hairbrush I would tell her that the staff has it for "safekeeping" at the desk and she can get it there. (have the staff keep extra "things" for your mom and they can giver her something telling her that they were keeping it safe for her)
Or you can tell her that you have it for safekeeping and you will bring it next time you visit.
If it is something important like eye glasses, dentures, hearing aids. there are ways you can track some items. Someone mentioned TILE there are other things you can attack that will enable you to locate them with your phone. Not sure how this would work with dentures and hearing aids.
Other than trying to convince mom that you have whatever is is she is missing there is not much you can do. If she is going to stress about something she will do so about anything. If this is a real concern is it possible that an anti anxiety medication might help eliminate some of the stress.

You could buy and attach the 'TILE' a device that lets you find stuff--like keys, wallets, kids, etc. My SIL is brilliant and absolutely incapable of finding 'lost stuff' He has a "TILE" attached to ALL his important stuff. I think the app to it goes on your phone (so if you lose your phone....maybe it also allows you to use ALEXA to find things?) I don't lose stuff, but DH does, and I am ordering him a bunch before his next business trip.

Don't know what the rules are at her NH. Seems like they'd welcome anything that would keep the staff from wasting time searching for lost stuff.

You honestly cannot help much. This paranoia is very common. Hopefully your Mom is NOT allowed to keep things of value in her room. Most places discourage this. While my bro, who went into ALF with probable early Lewy's dementia, stayed very very rational about almost EVERYTHING, he did get paranoid about loss of money. She has likely had the aneurysm from birth. While stress isn't good for anything I doubt it will rupture this aneurysm. As you know, any aneurysm can rupture at any time without reason, and it is more likely to with age and continuing thinning of the vessel. Life is full of stress, no matter our age; it is a part of living. Reassure your Mom by keeping her valuable and reassuring her she has nothing to steal. Do also know that this is one of the most common things the ALF deals with on a daily basis.