Menu button handling on Q5

Just a general inquiry at this stage.
Is there anything logged about the Q5 menu handling (that I have missed)?
The latest release of my app is responding incorrectly to the menu button : it brings up the watch's System menu, not my app's menu.
I don't have a device to test on but have a cooperating user, but the testing turnaround is a bit slow.

It works as expected in the simulator.

It was working OK in my previous release, and I have made quite a lot of changes, so it's probably my code, but I'm wondering what to look for that could cause the System Menu to be invoked from within my app?