TUI 737 APU question

[AeroSpark]

I have question regarding the apu on the 737-800. Every cockpit film I've watched the crew always shut the apu off after engine start and then don't restart it again until after landing. However the last couple of flights I've been on, with TUI, I heard a fairly loud whirring (increasing in pitch) noise from the back of the cabin just before pushback, which I took to be the apu starting, I then heard the same noise a few minutes before landing.
Assuming it was the apu I could hear is this a TUI specific thing to start it in flight, or is it a captains discretion decision when to start it?
Thanks

[Mr @ Spotty M]

The most likely explanation is that you are hearing the APU start prior to the ground power being removed, after push back the APU is used to stat the engines and would normally be shut down during taxi to the runway.
APU started just before landing to enable both engines to be shut down on stand prior to ground power being supplied, or to use on the ground for cabin cooling in hot temperatures.