Sensors on the HumanWare eReaders?

Hi again everybody, hope you're doing well. The subject line might be a bit vague so please allow me to explain a bit. I've had my eReader now for a few years and really like it. I have yet to use it for its intended purpose, i.e., reading material from BARD, NFB Newsline and BookShare. I've noticed a rather interesting behavior with it. Whenever I have it plugged into my Mac and come back from somewhere outside my apartment, the unit vibrates and beeps. It's as if it knows that I've re-entered the room or something. Is this just standard operating procedure, or is there a setting which I'm missing? In any case, I like this and thus far have had no issues that I know of with my eReader.