I would first assume this is simply trapped air down in the various media. This always happens to me when I cleazn my Rena. Sometimes it takes a couple days before all of it gets out.
If there is a seal broken somewhere, chances are the water would be coming out. Check all around the filter casing with your dry hand to feel for any escaping water.
Byron Hosking, BMus, MA
Vancouver, BC, Canada
Something we all need to remember: The fish you've acquired was quite happy not being owned by you, minding its own business. If you’re going to take it under your wing then you’re responsible for it. Every aspect of its life is under your control, from water quality and temperature to swimming space. [Nathan Hill in PFK]