In one case a mechanic was assigned to check the logbook on a through trip, while he waited for the passengers to deplane he did a quick walk around, which up until recently they were expected to do so there wouldnt be any last minute gate calls, and he found a hydraulic leak. He fixed the leak but the plane went out 20 minutes late. The Supervisor cited some new maintenance program that nobody ever heard of and put a performance advisory in his file basically saying he should not have looked at the plane. The message is clear, do not look at an airplane unless you are assigned to look at it. This flies in the face of everything we were taught since A&P school.