Emerson 5300 GWR Interface Level measurement issue

Kindly we have Emerson GWR 5302 for measuring interface level between condensate and water. The problem is sometimes there is no condensate in the vessel and only water is available. In this case the upper product thickness is zero. The echo curve is showing only one peak (surface echo peak) and there is no interface peak but there is some strange thing that there is interface measurement output coming from the device that has same value as surface measurement output (two arrows is in the echo curve in same location at the surface echo peak location). Logically, the interface level should be reading zero in this case but this is not happening, Is there any explanation why this happens?