Its common when you have a bunch of withdrawling addicts confined 24/7, and whose conversation revolves around drugs constantly to want ot use drugs in rehab. I have used plenty of times in rehab, some people in rehab are just not ready to quit and are angry for being there so ultimately use.
In one rehab I was at, outsiders from the rehab center would come in for 3 weekly in house NA meetings and 2 girls (from the outside) would come in with a few bundles (20-24 bags) and sell them to the people that want to use that are stuck in residential treatment.
I have seen OD deaths in rehabs just because someone needed that proverbial last hit (yea right) and tanked up on benzos, takes a bath, injects ODs and drowns in the hot tub.
Rehabs are a waste unless you truly want to get clean...
ANd some crazy shit goes on in those places regard less of highends rehabs or cheap state-funded ones.
Another rehab I was in at lunch time (we were not allowed to leave the building) a local dealer would show up and than he would have a tennis bacll that was sliced open and he would bounce it up to the 3rd floor, you would put your money in the ball bounce it down and than he would throw it back up with the amount of dope you wanted. 8(