Found this texture missing/ hole in the ventilation system.
https://www.youtube.com/watch?v=5ix0HBnss7c&feature=youtu.be
[Edit] map : bm_c1a1c at: -650 x, -2441 y, -1047 z
setpos -650.946655 -2441.151611 -1047.968750;setang -4.715991 85.363480 0.000000
You’re not even supposed to go that way
It is not a hole as it would leak, and it is a texture. tools/nodraw.
You just like exploring is all. & with that level of exploring I am sure you will find another area with nodrawed brushes, just as you can in any official Half-life. Game designers can’t seal off every little area, y’know? & the more they try the more restrictive the final product feels. Although they could have just aimed the vent upwards and applied the tools/black texture to the top piece, but it really is unneeded for a spot only 1% (or less) of the players will go.
Well I would consider nodraw as a texture, therefore my title, and I do love exploring 
You and me both! 
Good luck finding more.
They have done a good job by applying the texture in the final product, but wanted to know that does the black texture looking nice on it…??
Founded in 2004, Leakfree.org became one of the first online communities dedicated to Valve’s Source engine development. It is more famously known for the formation of Black Mesa: Source under the 'Leakfree Modification Team' handle in September 2004.