Nice research, I used no clip and got myself in the middle of the window, however I couldn't move in any direction, making me think this might not be possible, ill keep looking into it though.

So yea in pretty much all respects, it's an invisible wall, it seems to have the same characteristics as the barrier in the training room (which also doesn't show outlines and you can shoot through it).

I'm not sure what you mean by "doesn't show outlines" when referring to the spawn room. It's pretty clear that a playerclip brush has been put over that window section. Meaning player entities have collision with it, while other entities can pass through freely.

The firing range also has the same type of brush surrounding it, the only reason it's invisible in this instance is because it's tied to a func_brush entity, which means r_drawclipbrushes 1/2 doesn't draw it.

Bonus Fun Fact: You can get yourself stuck if you don't fully walk into the firing range during Soldier training. Most easily achieved by crouch walking through the entrance.