Today it struck me that not every encounter I roll for ambulatory monsters has to be with the thing in its "normal" form. It could be dead. It might just be some sign it's been there: tracks, waste, fur, blood, remains of a meal. It could be a warning that "Here Be Green Goblins"
Here's a quick set of tables I came up with while driving, and as such it remains untested:
Is the Monster in Here? Table
Roll 1d6 on this table after a monster encounter is indicated in a room (could also work for wandering monsters)
<=1 - Yes and Roll Reaction, What It's Doing, etc. Per Your Rules
2-3 Yes, but Roll on Status Table
4-5 No, but Roll on Warnings Table
6 - No and roll on Signs Table
Roll on the following only when specified above:
1 - 4 Alive, Roll Reaction, What It's Doing, etc. Per Your Rules
5 - 6 Dead/Destroyed (Roll 1d6, 1-4 Corpse is present and identifiable, 5 - 6 Gruesome Death!)
The next time a monster of this type is encountered, roll on the Is Monster Here table with a -2 modifier
1 - Waste / Excrement / Bodily Fluids
2 - Literal sign: eg. "Beware of Doppelganger", a human skull on a spear with a sign in crude handwriting "No Trespassing" and signed "The Orcs", graffiti, note scrawled on the wall by former victim/prisoner, etc.
3 - Tracks - foot prints, signs of slithering in the dust, claw marks, slime trail, etc.
4 - Howl, growl, a shout, etc.
5 - half-eaten meal / prey
6 - Incidental- shed fur / skin / claws, fabric scrap, dropped item
I got lazy and decided Signs are like Warnings, but with less of a chance of encountering the creature the next time they occur as a result on the encounter table.
So, roll on the same table as Warnings, but modify the next roll for this creature on the Is Monster Here table with a -1.
Like I said, untested, but I don't foresee too many issues with it, and it should help generate some mystery and elicit some concern from the PCs about what's out there. It can be used for dungeon stocking ahead of time, or in real time, as I'll be using it, when playing solo.