The radalt is a pretty good corrosion detector when it comes to that type of failure. Antenna to fuselage corrosion provides a signal leakage path direct from transmit to receive antenna; hence, the -8 feet indication.
The antennas should be dropped at the C check and the area cleaned. The belly collects a lot of crap, including that corrosive blue water.
-8 feet can be a valid altitude, as the 757 antennas are way forward, and will indicate about that when on ground. 0 feet, of course, is gear extended, at typical touchdown pitch.
GB