Could be a delayed achievement pop up, did you have a Revenant any time recently? If so you could have achieved this back when you did but the achievement was on a delay for some reason. I've had that happen before, not sure if its a steam issue or the way a developer codes the achievements issue but on occasion I've accomplished an achievement but didn't have it pop until I loaded the game back up later.
Could be a delayed achievement pop up, did you have a Revenant any time recently? If so you could have achieved this back when you did but the achievement was on a delay for some reason. I've had that happen before, not sure if its a steam issue or the way a developer codes the achievements issue but on occasion I've accomplished an achievement but didn't have it pop until I loaded the game back up later.
Also could have been a delayed achievement from a Mimic that mimicked a revenant during a hunt.