My mother thought things were being stolen too, though in mum's case it was her neighbours who were the culprits. She too phoned the police about it on many occasions. They were brilliant, being sympathetic while trying to persuade her to not to keep banging on the neighbours door. Mum once phoned the police to report that her false teeth had been stolen. When she told me about it, she said the policeman was trying not to laugh, while explaining that they were very busy and couldn't send someone to investigate immediately.
As for stages, before it started I was aware that things weren't quite right with mum but she was managing to live completely independently, and obviously hiding a lot of problems from me. She first mentioned stealing in the late summer of 2017, and the delusion escalated very quickly over the next few months. I moved mum to a care home in May 2019, as by that time she was beginning to be a danger to herself and to others, mainly because she started going out drinking with strangers in a local pub, and bought at least one man home. I think mum was in mid to late middle stages during that time. I'd now say she is somewhere in the early to mid later stages.
Does your mum have any help (other than you)?