Thanks, I decided to pretend it didn't happen - I know that no matter what I wouldn't tell anyone with or without malicious intent but if he has any doubt then letting him know I found it might force him to quit for no reason.
I thought about it for a while and considered worst case scenarios and came up with;
I mention my interest in growing and he trusts me well enough to tell me about his grow. Perhaps he has told a friend about it as well. His friend isn't as trustworthy as he thought and the guy brags to someone one night about how cool he is for knowing about a grow, and someone that hears about it robs my bro. Then he is stuck deciding someone to blame, maybe he asks the other guy about it first and he wont own up to it and suggests that it must have been me.
If he wanted me to know or participate then he would have told me, not waited for me to stumble on it and drop hints. Thanks to everyone for the input though!