Volvo would not start, then started by itself

My Volvo would not start after sitting in garage for a couple months without
being run.  I thought the battery was dead and called AAA.  While waiting for
them, my husband left the key in the ignition but did not turn it and did not
step on gas pedal.  He left the garage to do some other chores and when he
returned, the car was running.  He took it out for a drive with no problems.  
When we tried to start the car the next day, there was no sound again.