Slitherine

I'm not exactly sure how this happened since it doesn't seem to duplicate reliably, but I had a single trooper that needed commanding, I hit tab to jump to it, and it selected a square with explosions and body parts flying everywhere. After the animation ended, the idle unit selection vanished.

I am pretty sure he was killed by aliens rather than the enemy AI, which may be the difference (I didn't have many units die to alien offensives)

That sounds like he just got attacked as the turn started, I suppose he shouldn't be detected as idle if he's in combat.