You said in your above post that it died to a level 1 thief, not a warlock.
Thieves are stealthy. You need detect stealth to well, counter stealth. Thieves, dark elves, and traps have a chance to de-stealth thieves (and higher level creatures have detection and stealth too, but that's a different story).
EDIT: With the new patch now, thieves now destealth when they attack, so your situation shouldn't happen anymore