How did you do that?
The question mark is normally associated with a missing sprite error which in turn gives the missing sprite number.

The skele in question does not give any errors to me and does not appear to have any special worn items.
Did you kill it and did it have anything special?

If you can't see an enemy's HP, that is what it shows you. Or in other words, it's a guess as to what their stats are because you can't see them well enough.

LOL

OK I was thinking sprite bug because the only time I ever see the "?" is when a sprite is missing.
A long time ago I modified the client to show the "?" and also to only display the sprite number error once per missing sprite (to stops sprite error spam).

Looks like Ishtar shows it when he can not see to tell you what is there.