IŽll just hijack this for a quick a question :P
How you set up multiple functions for one key? IŽll go search for answer ->
Edit: Never mind. I was stupid.
Either Boom or MBF introduced the optimisation that monsters or players in the same subsector can always see each other. The line of sight check is skipped. (This makes sense, since a subsector is always convex.)
However subsectors don't necessarily end at one-sided walls and you can still be "seen" if you've noclipped through a wall.
You're always in some subsector even when you're outside of the map, but which subsector you're in is not well-defined. So it seems random as to whether monsters wake up or not.
In Zdoom and original DOOM, if you use noclipping move, the enemies never shotting you but for PrBoom... They shooting you! I think for some option command for this or waiting for the next version soon!