It was probably about 8" across, squarish-shaped, right? If so, it's the reservoir that the pump sits in. It's stamped into the bottom of the tank... tough to describe in words, but if you're looking at the bottom of the tank, there's a little pucker in the bottom of the tank below the pump... the metal plate on the bottom of the reservoir has a hole in it, and when the bubble in the tank bottom gets pushed back flat, it mashes itself over the edge of that hole, holding the reservoir in place. It's a goofy setup, but they've been using it for a decade (and it's less likely to leak than spot welds).
I've honestly never seen the plastic separate from the metal plate... that's an exceptionally rare occurrence. However, it can cause issues when the tank gets low, because the reservoir forces the pump to always sit in a puddle of fuel about 4" deep. Without it, you can still have issues on inclines, especially after it's been sitting for a while.
Might be time for a new tank...
-Joe