I watch Holby City, doesnt make me a doctor or knowledgable in their arena, and years of watching LA Law Rumpole of the Bailey unfortunately dont make me an expert in legal subjects. Indeed although a somewhat 'experienced' drinker doesnt make me an expect in brewing techniques!
All meant tongue in cheek, of course!
Think Rainboe & Topbunk have given you the answer, takes more than 20 secs for an APU to wind up! Have flown a few types and can assure you it is highly unlikely you'll hear the APU start up in a busy cabin with lots going on, you'll be hearing the air bleed being turned on!