You never know he could have been grinding it up fgor the next bowls but then decided to go home and forgotte to tell anyone.
Dont be to quick to jump to conclusions, people make stupid mistakes when there stoned n dont think houw it could look to other people.
But if he was doing it on purpose then just take bits of his weed when he has it and keep a close eye on your weed.
I always watch people when there next to my weed or fiddling with it.