Unless you have some accessories wired in to the key switch, and the accessories were on, then leaving the key in the on position will not run the battery down.
If your battery was weak before, and sat for a week, than the surface charge can bleed down to a very low level.
You should test the battery voltage by installing it back into the car (if you didn't already), and put your DVM probes on both main leads. Then with the key on, press the gas pedal and observe how low the voltage goes.
Anything below 9v under load, and you should buy a new 26G battery
Bob