I know that issues.
I do alot of things different than people recommend in their beginner guides.
One of those things is to hydrate the bulk substrate alot wetter than field capacity.
My tubs are covered with a piece of plastic bag for the colonization period.
Due to the extra water it happens sometimes that I got pooling water under the plastic, on top of the bulk.
Because I generally case everthing that doesnt matter, the case sucks up all the excess water.
Did you notice water drops on the lid of your tub ?
If so then it most llikely dropped down on the surface.
Is the tub modded with holes or unmodded?
Unmodded tubs tend to drop from the lid (ime)