Ohhh, boy, does this sound familiar!
We had this problem with-our difficult child, in regard to our easy child daughter, a neighbor, and the neighbor's daughter.
difficult child still cries about it (the police got involved).
Part of it is the physical/sensory part. with-difficult child, it had to be expensive Victoria's Secret sexy lingerie. That took me out of the picture right away. I can't do lace.
But also, we found a note, 2 yrs later, that listed the people's names and their attributes: tall, blonde, sexy. He was right. He does have good taste. But that's beside the point.
You do not steal. You do not invade other's privacy. You do not sneak.
I bought him lace panties.
He hated them.
They had to be associated with-a particular person.
The therapist helped a bit, but I do not think we ever totally resolved the issue. difficult child kept stonewalling. I did not go to most of the sessions because it was too much to have "mom" in the room. But it was very lengthy.
We gathered a bagful of panties and brought them into therapy (I hid them in a big purse, and then we blindsided difficult child. Do not do this with-o a therapist or it will backfire.) We had difficult child talk and cry, etc., and then had him walk, alone, outside the therapy bldg and throw the entire bag into the trash. Sort of like a ritual.
Part of our solution was to put locks on the bedroom doors. Another was to call the police. Another was to buy gift cards for the neighbors to replace the stolen goods, and have difficult child write a note of apology. I showed him how much it cost, and made him write the note. He could not deliver it because he was banned from their property, but it all worked out for the best. They are sympathetic to him now (we told them about his Asperger's) and no longer afraid.
Part of it also involved easy child directly confronting difficult child and telling him how much he invaded her privacy and hurt her feelings. She is his idol and he was crushed. Also, the neighbor's daughter was one of his best friends and that went down the tubes.
This is a fetish and part of repetive behavior, as others pointed out. It takes a lot of work but all is not lost.
You have to be patient, which is not easy.