It didn't have the grey dot in the direction it was facing? Looking at the tooltip of the archer with shift, did it say it was sleeping, or anything unusual? If your character had a high enough dexterity, you may have been able to walk in front of them in the dark caves without them noticing you at all.
I think I just ran into what DeadStack did, it was a miner in the dwarf fort with no dot facing indicator, I shift-looked and it didn't say sleeping or any status other than what's in the tooltip by default. I reloaded the save and the glyph for the miner entity was gone, but was now appearing in the spotted creatures list on the left no matter where I looked. Artur I'll zip up the save, let me know if you'd like me to send it to you.
