I was wondering the same thing every time I read a post on ABS failure, thinking if it gets some use (the ABS system) every week or two, it would be beneficial in keeping the seals, valves, pump etc. "limber".
Not a problem here in the ice covered mondo tundra...it gets a workout in the winter at least.
In the more humid months, I've seen water (I assume) floating on top of the brake fluid in the res. & removed it with a spoon that I bent the handle so it looked like an "L". Then just top it off.(note to self: don't eat cocoa krispies with that spoon)
Probably should do another flush this spring before Moab.
I'd flush that mess asap brother. Wouldn't have been water, it goes to the bottom. I'd run a quart thru there...at least. A Mitivac is even better...just always pull from the wheel farthest from the ABS pump