Ok, this was a definite "What the...!?" moment:
Forgotten beast shows up (Forest Titan), at top/embark level, body made of sand, nothing too special, but I send out a squad to deal with it, and they're all standing around the edge of the water moat, so I figure they killed it.
I look down and it's stuck at the bottom of the moat (which is river fed) doing nothing. Ok. And still... doing nothing. Meh, can't reach it to kill, it won't move, but whatever, it has no line of sight to anyone or anything, so no harm no foul, I have a new Forgotten Beast aquarium.
Second forgotten beast shows up from the second cavern level, and it's a hornet, so I send a squad to station at the top of the stairs below the dining/living area, and the second it pops out, they slice it into several pieces.
Then an elven caravan shows up, so I'm off to the trading depot when I get a message:
"Dwarven Child cancels Eat: Interrupted by Forest Titan."
Wait a sec... there's only one place these dwarves eat, and it's not the river! It followed the river all the way from the moat, down the fresh water supply shaft, and came out a well ! (I have a row of wells for fancy water supply zones in the dining hall )
Of course 60% of all the military was sitting there having lunch, pulled out their shiny (just forged) adamantine battle axes, and had it's head off in about 3 seconds.
But wow, I am just amazed. A sand creature makes it through the water supply, comes up a well, and attacks the dining hall!? GENIUS!