Wow, I hope to hell you got a policy about random drug tests or If I was one of the 11 I would be calling my lawyer buddy right now just for a violation of my rights. I'm innocent till proven guilty, If your policy don't support randoms then your just being a prick.
I honestly think your going way overboard. I would make a big meeting about it. I would express the ramifcation if busted but that would be it.
So some guy got stoned. Did the pizzas get done that day? Were there late pizzas? Was anyone hurt? Was there any loss in productivity if the answer is no to all of them then your just wasting your time and the company money since productivty was there.
Most of the bad ass chefs smoke. Period.