You have my heartfelt sympathy - there are no good answers here, just coping strategies that each may work for a while. This type of paranoia is typical of some types of dementia.

When we were helping my great aunt, we got her to her doctor who placed her on meds (sorry, which one is lost in the memory bank) that eased this. This worked until she decided the doctor was "in on it" and quit the meds.

Whenever she called for help, either I took her to a local restaurant to talk about it, or my husband (often the rescuer) had her write it down so "they" couldn't hear. With a list, sometimes I was able to locate her missing items.

Please don't completely discount what she is saying; we had an elderly neighbor we kept an eye on, and it turned out a caregiver was taking some things we had to contact the (faraway) family and get her fired.

Do you think Mom can/will write down what is being taken?