If you climb up to the ventilation tunnel with boxes, there is no wall.
version : 1.0.1.0/14 4104 insecure
map : bm_c1a1c at: -632 x, -2217 y, -912 z
That’s the “wide-testing” by comunity 
. A lot of us doing strange things or trying stuffs which are not usually done by “regular” playtrough :o) I have to try everything and explore every part of the game usualy… I’m sure my gameplay will last much much longer then 8-10 hours ;o) Simply enjoying all the aspects of this amazing masterpiece!
Put an easter egg or just some enemy and a dead end vent in there.
Don’t discourage player exploration. :[
Wouldn’t a nodraw texture into the void create a leak with light?
My vote for an easter egg, instead of just blocking the access, or adding a wall at the end, maybe junction going up, or any other side, but with bent ceiling preventing you to proceed any further in… Or something else that might be considered to look realistic.
Nodraw seals the map like any other normal texture does.
IIRC in HL1, that vent gets closed off by a collapsed ceiling in the vent.
Another vent who you can go back is the vent from the AM Lobby to the control room, if you put a chair and climb the guard dissapear and you can go to kill Eli and Kleiner with the crowbar