This is a message that Hammer is throwing at me in a Source Engine MP configuration towards Garry’s Mod.
Looking for a solution to this I would only stumble upon threads where other messages are investigated when a full log is posted by other individuals.
I have compiled multiple map source files provided by Source SDK, from various test maps to a map that was a bit bigger, the Half-Life 2 loading screen map. I did not attempt to go bigger than that as it will probably overload me with compile errors due to legacy entities used on those maps.
I have not been able to replicate this message in the compile output on any of these maps. I have also cordon bounded my map to small areas and even then the message still pops up.
Although it does not seem to affect the output of my map, I am still a bit annoyed by the fact that I can not pin point what on my map is exactly causing this, especially with the above research performed.