I’ve seen this “Easter Egg” in the 2012 Mod version for a while now (several years), and I don’t know the meaning of “CM”. Thoses letters can be seen in a elevator in the chapter “Office complex”, in map bm_c1a2c in Steam version (coordinates 1907 -749 507).
Likely the initials of Carlos Montero, one of the developers, similar to the way Dave Riller put his last name in the original game’s On a Rail chapter.
Thanks you ! I was thinhking about it, but I wasn’t know.