[tptacek]

From what I can tell, there's nothing remotely "clever" at play here. The search context we're talking about is many decades old. The arrest that instigated it was not subtle; any of (i) reckless driving, (ii) driving without a license, or (iii) DUI can get you searched "for reals". In a "for reals" search, your phone is not off limits, never has been, full stop.

If you want to force the situation into a grey area, use a strongly encrypted passphrase protected phone, like an iPhone. You'll have a lot more room to maneuver in the case where someone tries to compel you to divulge your passphrase. That's not what happened here.