Nameless Seer nowhere to be found..

https://gyazo.com/ffe24e18bae78166ef8b5bb9428329dc

This is the very first time this has ever happened to me. All monsters are slain, including the ones that should spawn him, 0 monsters remaining. I've ran through the entire map inside out 5 times, not a sight on him.. Has anyone encountered this before?

Is there a lab trial or hideout in the map? Seer can spawn in there, unless that's been fixed recently.