The way i see it, there is no need for abstraction, what about scouts actually entering map and, well, scouting?
Basically: Scout enter map, he then chooses Exit map tile and proceeds to walk to it.
He will be stealthy and will not unstealth to initiate fight: he will also try to avoid dogs and other "stealth detectors".
Should he find something interesting in his los, he will investigate it (=move to it and look at it) and eventually draw conclusions:
Finding trampled saplings/dirt. ("somebody was in here recently")
Finding post-battle junk and other items lying around (bones, chunks, clothing and armor) will give strong "somebody might be settled nearby", in case of battle derbis "we have to send larger scouting party"
Stumbling upon construction - different types, dyfferent conclusions: bridge does not necesarily mean there is civ site nearby, but bed/stockpile/workshop is strong suspiction.
Finding actual civ unit will prompt Scout to follow it untill he sees more, should he follow him to fortress entrance where he can see chained dogs and others, it is civ site, allright.
Basically, what you would expect from scout.
Scouts will be more frequent when you are close to settlement. When scout leaves with strong suspiction, he might return (with small group, with ambush party ...). If scout gets "missing", lots of this can happen: less scouts, more scouts, scouting party, depending on civ from which he was. (elves might send more scouts, humans larger party, goblins might not care)
Also, "misunderstandings" can happen: scout can enter map where you are just ambushed by goblins and see only goblins (and maybe Kivish Woodcutter corpse) and figure out that there is Golbin tower nearby. Season later: !SIEGE! "Oh, you are dwarves? Not goblins? Say, we are at peace, right? Nevermind them, we will be on our way."
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Important new features:
* Hidden doors, stone doors above certain quality can be designated and "hidden", looking just like stone to someone who does not know it is there and who fails "stealth detection" roll.
* Stone walls made to resemble undug rock, ditto for floodgates and floors.
* Hidden levers.