Teams are apparently tipped off in advance of drug tests because the testers call ahead and ask for parking passes and to have a room set up for them in which to conduct tests.
My first impression was "eh, no big deal," because, hey, the guys gotta park. But the article does a good job of comparing and contrasting how the NFL handles this -- their testers get permanent parking passes and a dedicated testing area and can literally just show up with no warning.
There's a hysterical quote in there from some doping expert about how this is "scandalous." I don't agree, and I question whether this loophole has ever actually tipped anyone off. It is a problem of perception, however, and one that seems easily fixable, so why not fix it?