I didn't loo'k', but I did see discoloration in the brook which I assume was a cloud.
The funny thing is that I usually take the damp stone warnings seriously too. But I wanted power right at that level! I bet that was my problem -- I would be unsurprised if the words "what's the worst that could happen?" went through my head!
Incidentally, this was my first fortress flooding incident. Great fun!
(well, my second, but the first one didn't really count because all that got flooded was a couple magma furnaces/forges in a room that I hadn't even stocked with supplies. Although it was fun discovering the third cavern layer from underneath a lake!)
An update: after failing to stem the flow and writing off my first years work as a lost cause and starting to make plans to dig out a new fort, I wound up noticing a choke point I had missed, and I managed to build a wall before it was too late! (and then build more walls to stop the other entry point I had missed as it suddenly started filling up)
One batch of temporary housing, my hospital, my stone processing center, and a stockpile with a set and a half of steel armor are going to be a lost cause until I'm bold enough to actually stop up the brook, but I've saved most of the rest of my fort! My metal industry room came out untouched, and it was painless to drain my newly dug out (but unfurnished) large housing complex into the caverns below. (as well as my food stockpile!)
(Now that I've said it's painless, I'm probably going to get a nasty flying FB enter my cavern and fly up my drainage stairs. I really should put some grates there, but where's the fun in that?
)
In the end, I may have actually been helped by the flood. I had thought my metals were hematite, magnetite, and sphalerite. But I found a galena deposit in my drainage tunnel! Yay silver!