GarageYears,
if you know how the signal looks like you searching for, you get much more signal to noise than with only digital reducing the ambient noise. However the current frequency tolerance and ping time interval tolerance are too sloopy to use this technique effective. My point is, that this can be improved very easily without adding much cost, weigt and volume to the pinger. Think about it as you know the exact conversation you where looking for in your example beforehand.
That technique is the only reason that GPS works with relativ small receive antennas and low transmitt power in the satellites.