I get asked to unlock my dev laptop every single time I go from the US to Montreal. The TSA person sits there and waits for my WM to boot before waving me past.
It seems more like they're trying to determine that it is in fact a laptop and not something resembling one.
That's been my experience as well. I've visited Sydney twice, and both times I've been asked to light up my devices. Granted, I was on work trips requiring three separate laptops which does probably look suspect, but once they were booted they did not request to browse anything and were satisfied to see them working.
All the more reason to dual boot into a decoy OS. Does not stop a targeted investigation, but lets you pass a cursory examination where some thug might want to rifle through your data.
Edit: now I am gleefully thinking about how I would craft my decoy desktop persona. What gives me the most effective non interesting profile.
Might also be documenting serial number and identifying radios associated with your device. As a dragnet etc. Palantir and co love them some massive data hoards!
I've only had it a couple of times, BIOS was enough. Very good as the battery in that machine was shot and it couldn't actually boot. (I always used it plugged in, it wasn't worth the cost to fix.)
They are looking to see if you've gutted a laptop and filled it with explosives or drugs. Although tbh, a computer that can launch a desktop doesn't need much physical space and the battery just has to last long enough for a cursory glance.
It seems more like they're trying to determine that it is in fact a laptop and not something resembling one.