Last Wednesday, I plugged in my rechargeable battery charger and two rechargeable AA batteries into an outlet at work. I haven’t been to work since then (circumstances beyond my control, blah blah blah). When I get back to work Monday morning, am I going to find the charger, the batteries, and/or my office ruined?
The charger should shut itself off when the batteries are fully charged. At least, that’s been my experience with such devices.
It probably won’t catch fire or explode 
Depending on the type of batteries and charger, you may have done some damage to the batteries. Ni-MH cells are less happy with being over-charged than Ni Cd cells, but all but the most basic chargers will offer some form of protection. Some chargers will have a simple timer and then either switch to trickle-charge or off altogether. More sophisticated chargers constantly monitor the state of each cell and adjust accordingly.