I didn't know either, i've had small power banks for years before they became popular, i suspect these rules have only been put in place now that everyone has them. Probably just going to play ignorant if I do get asked. Not sure how they would have noticed if the powerbank was in his bag or whatever.

I do find though the flight attendants are now alot more "technology aware" ... e.g. I was on a flight recently and they asked everyone to put all mobiles and headsets away and out of sight until the plane had taken off, which i've never been asked before. Also have seen flight attendants creeping over peoples shoulders before takeoff to ensure flight mode has been switched on, whereas before all that was just something they requested people do but didn't actually follow up on or check.

They saw a cable running from his laptop and into his bag. As you say, flight attendants are pretty on to it with technology and I suspect they are looking out for people doing certain things. Using a power bank is likely one of those things.