Sound from 1 fret back?
 

When I hammer on the 2 highest strings down below the 8th fret or so I can hear what sounds like the next note back. It's like the back side or the string (behind the finger) resonates on the next fret back. Not very noticable but it's there I hear it on all 4 of my basses. What causes that?

These sounds come from the length of string between the fretted note and the nut. They are actuated by the force of the hammer-ons the same way the as the string length from the fret to the bridge. Obviously, the fret to bridge length is the portion that is "heard" by the pickups and sent to an amp.

If these sounds are very obvious and bothersome, the usual causes are a neck with too much relief, a nut slot that is too high, or a combination of both.

That's just simply string resonance and since it is behind the note fingered on the neck it is not picked up by the pickups. But you will hear it when playing at low volume from the neck or without being plugged in an amp.