Your scout has died of attrition

[Browd]

Don't recall for vanilla, but in G&K this will happen if your unit sits next to an enemy citadel for too many turns (-30 HP per turn).

 

[IkM]

Happened to me too. but first, I didn't get why they took -30 damage then realized because of citadel. they took -30 for 2 turns. they almost get killed.

 

[bdporter]

I've only seen it with Carthage ending turns on mountains.