Brain cells acquired, turning my comment into an answer:
The first thing you should do in a situation like this is to open the container and press the vegetables back down. As fermentation occurs, the active bacteria produce gas as well as acid. The gas will get trapped in pockets under the vegetables, and once enough pressure builds can lift the vegetables out of the brine. When the ferment is chilled, those air pockets will shrink, just a tiny bit. But due to friction or other forces, the vegetables might be stuck in place, and not be able to sink back into their original positions so easily. And so the brine "sinks" to fill the space instead, which causes the appearance of vanishing brine. Ideally, you shouldn't add anything to your ferment once it's under way (unless you're specifically doing a secondary ferment, which is totally different), so attempt to press first before you top off.
But if you do need to top off the liquid in order to keep everything submerged, always use a brine rather than plain water. Your fermenting vegetables are tightly packed, and adding plain water on top could give other undesired bacteria a chance to multiply before the existing salt/acid can diffuse through it. Not to mention that the plain water is going to leach salt away from the top layer of vegetables first, which is more likely to give you uneven results.
If your ferment doesn't call for a brine, a ~3% brine solution is generally reasonable, but if you dig a little you'll find plenty of recommendations.