I don't know. Just a couple of questions:
-where is your second key? In a location where the panic button may be pressed?
-could it be that, if the key remains in the ignition but the car's not "on", it may be "talking" with the ignition and is therefor discharging; then the battery may require replacement? (Symptom would then be quick repeated beeps, about 9 of them, according to the manual.)
Good luck!