What would happen is that it would stay in the tank as ice, until it melted when it would stay in the tank as water. It would then be found there (as either ice or water), at the point that the investigators looked at the tanks and, in fact, found "no significant quantities of water".
From my reading of the AAIB report, they tested the wing tanks, but due to contamination with foam, etc, the center tanks were not tested. Also, due to the center tank breach by MLG, does it not seem possible that any ice may have melted and drained from the center tank by the time the AAIB had a look, or any remaining water mixed with foam etc so as to make any analysis difficult at best