I just printed something off on the college printers and left my wallet on top of the printer.
I cam back not even 5 minutes later and it was gone. Turns out it was behind the printer and someone had taken all the money out of it (80 Euro).
My point is ALWAYS make sure you have everything. I am a person that is very careful of everything, after I do anything I make sure I have everything I check my pockets after I leave the door, bus, school, get out of a classroom. Despite all that I once forget my wallet and this happens.
Just make sure you always know where all your stuff is and even if you go away from somewhere for 2 minutes, unless your trusted friends are staying there like a classroom or a break area where you use your laptop or whatever, ALWAYS take it with you.