This has happened with a few batches now. My three-piece airlocks are drained of the sani inside them repeatedly. Even after a few days, when fermentation doesn't seem as rapid (and therefore the temperature of the beer shouldn't be that much warmer than the surrounding air), I'll add more sani, and it gets sucked right through. Thoughts?
There are only a few reasons why this might happen.
Suck-back due to temperature changes
Airlock is damaged
Somebody is messing with it
As it has been mentioned by others, s-locks are better at keeping liquid, but are nearly impossible to clean if you get a blow-out. Three-piece airlocks are easier to clean. I use mostly 3-piece, except for things like mead, which might be fermenting for months.
I keep my airlocks filled with cheap vodka. That way if there is suck-back, it adds a microscopic amount to the beer's abv. Plus I don't have to make sanitizer or worry about the effectiveness of the sanitizer in the air lock.
Pay careful attention to how far into the neck the stopper is inserted, and (more importantly) if it is moving at all. If it's pushing itself out, then that will in turn suck liquid into the carboy.
This happens pretty frequently when first inserting the airlock + stopper. I've gotten in the habit of first inserting the stopper, taping it in place to keep it from sliding out, then inserting the airlock.