I'd have stolen his backpack, but left the macbook and other contents there. Then mention some stuff was left by someone to security while leaving.
Then the guy comes back and gets to have some very mixed emotions of being happy his valuables are still there but confused as hell why the backpack they were in isn't. And hopefully this would trouble him for a good week or so, which would mess with his head and on the bright side probably make him less likely to forget something like that again.
Or alternatively if I didn't want to rip them off for the backpack or was just too lazy, I'd slip in a note saying "I have stolen exactly one item from you", and have them go mad wondering what it was.
Then the guy comes back and gets to have some very mixed emotions of being happy his valuables are still there but confused as hell why the backpack they were in isn't. And hopefully this would trouble him for a good week or so, which would mess with his head and on the bright side probably make him less likely to forget something like that again.
Or alternatively if I didn't want to rip them off for the backpack or was just too lazy, I'd slip in a note saying "I have stolen exactly one item from you", and have them go mad wondering what it was.