Here is a simple solution:
915LM - Garage Door Monitor
My Dad put everything in his pockets including the garage door openers. Needless to say the door often opened when he sat down or reached in his pocket. Rather than depend upon his neighbors to tell him his door was open we got one of these and placed it in his line of sight from where he sat most of the time.
This is no longer an issue now that he has moved to an apt (car is gone finally too) but makes it easy to check when we are cleaning, checking on or painting at his house.