I remember reading on a different forum of some guy who would smoke at starbucks on his break, and alot of other people started doing it as well. After a few monthes managment caught on and all though they never actually caught them in the act they knew what was up. So they talked to coprate and they started drug testing. Im not sure if they do it to every store it might just be that one.
starbucks of course has the legal right to drug test their employees if they care to do so, as well as to check belongings on entry to the store and before leaving. these are both very rare things though and not commonly practiced. i've never known anyone to be tested simply because they've been hired, or at all.