I kinda doubt somebody just came up to your random locker and popped the thing open with a screwdriver. You have to have left it open or not lock it properly.
And yes, every night they do a count of the locker keys to figure out which ones are missing. They will take a masterkey and open it up, remove the contents, and a new lock is installed. So if on Monday you buy a locker and save the key, it wont work on Tuesday.