Here's what I do if some one finds out about my grow(I don't tell anyone...need to know basis)
I tell them your the only other person on this planet that knows about this grow,If anything happens your the first person I'm coming to see!
It works well.People are afraid to talk about it to anyone else in fear of getting blamed for someone elses robbery.
Just when you say it be nice and none threating ,that way when you say your the first person I'll come see ,with a big friendly smile on your face.They think your nuts...Also I'll give them insentive to keep the secret."If you don't tell anyone and nothing happens to my crop I'll kick you down a fat sack(never say exactly how much...this keeps there imagination running to the sum of a huge bag at harvest)...
Trust me this is good advice because sometimes someone will stumble into your room or show up at a wierd time...Or professionals like the stoner bug guy I imploy for all my outdoor and indoor pestiside spraying...I tell him everything that I said above and then kick em down a 1/4 or something after harvest.