ai_debug_los
ai_debug_los is a console variable available in all 
Source games.
A cvar that is used to inspect what (apart from ordinary world brushes) blocks line of sight for NPCs. When active, these obstructions will be highlighted with white bounding boxes.
- ai_debug_los 0
- Do not highlight anything. (Default)
- ai_debug_los 1
- Highlights all things that currently block line of sight.
- ai_debug_los 2
- Highlights all things that can potentially block line of sight as well (like disabled brushes that can be enabled).
This variable is a cheat.
