Yeah, I think you're coworker is probably correct. I thought you were saying that there was a small bump in the plywood, but if the "bubble" is over the whole length of the top, it's much more likely that the frame you're attaching the plywood to is warped. 2x4s really aren't all that straight or dimensionally stable. if you have a 36" straight edge you can place it on top of the plywood, across the front and back width, and diagonally corner to corner. Then I'd loosen the screws holding the plywood to the frame, shim the plywood so it will be secured flat, and then retighten the screws and check again with the straightedge. Might take a few iterations to get it right.
Nate