Game Mechanics: Players can evade yellow text somehow (without zoning or plugging)

[Videri]

Twice now, I've killed a PvP opponent above level 10 without yellow text. I killed them, they left a corpse which I could not loot, and no yellow text was displayed. Has anyone else seen this?

I speculate that they DoTed themselves with some clicky or potion, or more likely took off a big +hp item(s) at the right moment in order to prevent my attacks from reducing them to 0. Is this what they did to prevent yellow text? If not, how are they doing it?

Both times, it was a rogue; not sure if this is incidental.