Consider a "last scouted" overlay

[sprunk]

A shader overlay (like those F1, F2 etc modes) that would be somewhat like a saturating and very-slow fading LoS. An area currently in LoS would be fully bright, an area last in LoS (say) 2 minutes ago would be half dark, an area last seen 20 minutes ago would be dark.

If it is to exist perhaps it would be ideally live in engine, since it sounds generally useful (and maybe a nice example of a custom LoS based shader).