Leaving your backpack sitting around is a good way to get it stolen. If you don't want to dedicate 100 per cent of your brain power to watching your bag, Instructables user bergerab shows you how to build a small anti-theft device for you bag to sound the alarms if someone tries to steal it.
When the system is activated, the device senses when it's picked up and sounds an alarm if it's moved too much. The whole thing is powered with an ATtiny85 and a few cheap parts. It's a clever little system that should help give you peace of mind.
Backpack Anti-Theft Alarm [Instructables]