`info_ambient_mob`

This entity is Obsolete. Its use is discouraged, and it may only exist/function in older engine branches.

This entity is not in the game by default, but got left over in the FGD.
This feature is not available anymore.

info_ambient_mob is a point entity available in Left 4 Dead 2.

…

It is a universal ambient mob start/end (use with care!).

Uses in official maps

The following maps use info_ambient_mob. Targetnames are not used. (these don't exist in-game but are present in the compiled map)

Located in a secluded area behind the building right after exiting the safe room. It is the area furthest from the bar that has a dynamic pathway in c5m1_waterfront.
An area of concrete fences unreachable, between the gazebo and the narrow pathway leading to the queuing/quarantine area.
Facing in the direction of the escape route, way behind the quarantine/queuing area before the gauntlet event. Behind concrete barriers and fences by the weapon spawns along the highway path.
Behind concrete barriers left of the stairs leading to the park entrance.
In narrow alleyway from the previous map, right of the building when entering the safe room in c5m1_waterfront.
In flow towards park, it is in the park to the far left where it is unreachable to survivors. It is square space consisting of walls made out of bush props.
Right by the park bushes (number 6, above), but unreachable and surrounded by concrete walls.