This isn't uncommon. Do look it up under Dementia and Paranoia. It is very common that when we begin to lose our faculties we fear people are doing things they should not be. Affairs. Taking money. Stealing from us. So common was it for my partner's Mom that she accused her dear housekeeper Wilma all the time. It got so funny to us that we would say, whenever we lost something "Wilma took it". Good luck.
Has your husband been diagnosed with dementia?
Paranoia can be very common with some dementia. Having him seen by a geriatric psychiatrist might be a good step to take.