Think I accidentally found two "features" of expedition mode.
I was using it in "prompted" mode, basically as a way to sleep the device while experimenting with various things throughout the day - ie. a half-way house between constantly powering it down and then rebooting 15 minutes later to try something else, and just leaving it on all day (and yes I could've also used the Battery Saver mode to just turn off the screen, but didn't).
Firstly I realised that upon holding the power button to bring up the prompt, then selecting Expedition Mode, as well as sleeping this actually initiated an activity recording. I sort of get this - normally one would only put the device in expedition mode if they wanted it to be recording in the background, but even so it surprised me - I would have expected to have to either start an activity recording as normal before entering Expedition Mode or perhaps for a prompt to come up ("Start recording?").
Secondly I discovered if the device is resumed from Expedition Mode as usual with the power button, but then no other button is pressed, the same prompt to re-enter Expedition Mode as per the Auto-mode appears and then, assuming no user action taken, it automatically re-enters Expedition Mode. I presume this is a feature to counter the power button being accidentally pressed while the device is stowed, but I hadn't seen this documented anywhere.