Limited support
This feature is only present in Portal 2: Community Edition and Portal: Revolution.
Caution:

SRCIndicator is an internal entity and should not be used on its own.

BASE SRCIndicator

Adds an Indicator Name option to toggle overlays.

KeyValues

Indicator Name <target_destination>

Set to the name of a set of info_overlays to toggle when this is activated and deactivated. The name may also point to a prop_indicator_panel, which will also be toggled appropriately.

Inputs

SetTextureIndex <integer>

Manually change the index of the overlays. prop_indicator_panels must not be used.