In my time keeping red paper wasps, I’ve notic…

In my time keeping red paper wasps, I’ve noticed that in open cells, there are often left drops of what appears to be some form of nectar or honey-like substance. It is not nearly as viscous, and appears to be mostly water. I’ve seen it in both the larger and smaller nests in my nuc box, as well as in the nest in my window.
Anyone know exactly what this is and what purpose it serves? I can’t find anything on the subject online.