Thanks for your responses
Heh... I was in fact in the very last seat right in the tail.
The APU most definitely started up after the shutdown of both main engines, which was after the lights flashing / thrust increases etc. I can tell the difference between the sounds of the two main engines, the APU and aircon packs...
As an experienced SLF
I notice that the most "usual" thing to happen is to arrive at the stand, aircraft stops moving, small pause... engines shut down and then seatbelt lights off. (probably a whole lot of other work going on in the front office that won't get noticed in the back)
On this flight I noticed a different sequence of events and I was sufficiently curious to ask in this forum (hopefully in the right place)
Thanks again
Rich