I know in some 727s the APU being on generates a T/O warning horn and on others it does not.
I believe the newer 200As had that feature. We had two 100s and an old straight 200, all had the -9 engines. We did lease a ex-Air France 200 with -7 engines, now talk about a dog. I hated to be assigned that aircraft while we leased it, but it was cheap.
On our 100s, the start/stop switch was disabled after airborne. As I related when the FE first tried to shut it down, he used that switch and nothing happened. The APU only shut down when the fire handle was pulled. It would not surprise me if on the newer 200As the switch could still be active after takeoff.
By the way, one of our 100s had the additional baggage fuel tanks. We had about seven hours of fuel with a maximum load of 35 passengers on board. It's been over 12 years since I flew that aircraft, or any 727 for that matter, so I cannot recall just how many extra pounds of fuel we could carry.
I much preferred flying the 100s compared to the 200.
Also, I'm not sure about your particular aircraft or the way it was equipped, but if the bell rang right at rotation it may be that the APU was wired in with the WOW switch to generate a bell if the APU was on at liftoff,
No, the fire bell did not sound until we had been in the air for about 45 minutes, on both occasions. On our aircraft there was no tie with the WOW (we called it a squat switch) and the APU. Now one other thing now that I am thinking about this, on both flights they were short hops and we never got above FL-250/290. That could be the reason the APU did not flame out.
Not being able to own up like that is one of the biggest red flags for me in terms of a crewmember.
That was not the first red flag we got from this guy, but we were a government operation and he was ex-military, so what can I say. He's an MD-80 Captain now for the same operations.