One thing that I have found that can make the map seem very quiet by glitching the encounters is blocked off areas with access to the area they invade from.
For instance, in one of my older forts I never had any ratmen attack, and thought it was odd, then one day just for kicks I decided to take down my stone aquaduct across the chasm and replace it with a solid platinum one. When I stopped the water flow and opened up the area, massive swarms of ratmen streamed in for years. They were a constant series of attacks, every time a dwarf would be near Id have a pause every couple seconds with more ratmen.
I lost several powerful (and very well equipped, all high quality/masterwork steel plate and weapons) soldiers just to the sheer number of them - theyd be in a 1 wide hallway, but the ratmen would pile onto eachother (and the dwarf) and wrestle him. He'd hack apart 50 or so and then get overwhelmed and slowly be torn apart limb by limb. His buddies would pile onto the same spot and slaughter another swarm of them, but they kept coming and eventually I had 6 of my best guys overwhelmed and mangled. 3 of them died immediately, one was bedridden permanently (mangled upper spine? I guess broken neck), and the other two died a bit later. In the end, all my refuse piles were full, and I had God knows how many ratman parts all over the entire hallway and aquaduct. It had to be in the neighborhood of 1000+ ratmen - Years later, I still have hundreds upon hundreds of ratmen bones scattered around, and I have had 2 bonecarvers going at it constantly. I made a squad of legendary marksdwarves with all the bone bolts
After it eventually stopped, I kept the area open for a bit, and the ratmen started the normal invasion timings, at that location exclusively. Not sure why it was counting it as the sole invasion point, since I had wells and other points of access to the chasm - dumping areas and such.
Anyway, if you have anything like that, you might try opening it up and seeing if you have canned encounters.
Dwarf-slaughtering ratman whupass-hordes conveniently packaged into aquaduct tunnels. What more could you ask for?
I even got an artifact ring out of it showing dwarves making plaintive gestures, ratmen laughing, ratmen falling, and ratmen dieing. Must be a big ring.