I believe it’s a painted cinderblock support column that protrudes beyond the wall slightly. There are four others visible through the door on the left.
Looks like it has been cleaned up to a state before the Resonance Cascade, for cinematic purposes.
Not a bad idea, considering I haven’t seen many maps around based on Black Mesa in a clean state.
I have cleaned up the gore, fixed some lighting, fixed broken parts of the map, combined part of another map with it and sealed it off so it is playable and doesn’t load onto the next level.
in other words, tried to make it look as if it was before everything had went wrong in black mesa.