Are there motion sensors appropriate for multi-generational homes?

I live in a 8-member, multi-generational home - from my elderly parents to my ten-year-old sons. My dad likes to get the mail but should not go alone due to risk of falling. He doesn't remember (or want) to tell us he is going, which is a huge safety concern for me. I am interested in a door-sensor that will alert us to his leaving - but one that will not 'ring' when anyone else leaves - maybe a 'trigger' that I can attach to his cane. Is there anything like this available?