I killed a Suspicious Chest on an off-turn.
I think I have done this before, but then there was some kind of Damage over Time. This time, there at least appeared to be no DoT, even though I’m pretty sure I had fired Colen – as the Chest had spawned next to a Guardian Gazelle, who had hit it with Dance of Spirits.
One possibility would be that the Dance of Spirits failed to protect the Chest from Colen’s DoT (which failed to appear as icon, too) … but I suspect Colen is a red herring here. I don’t even recall if I fired his special during a vulnerable turn – I figured that what with Dance of Spirits going, there wouldn’t be a vulnerable turn for some time.
The other possibility I see is that, somehow, the Chest started taking damage (at -50%, perhaps, per the skill description?) while under the effect of Dance of Spirits.
I suppose I should also note that the Suspicious Chest, disappointingly, did not have a dancing animation. Sloppy! But I did verify that the Dance of Spirits was in the list of effects in the pop-up, when I noticed the timing worked out so that Guardian Gazelle’s special went off just as the Chest was “vulnerable”.