If I am to understand the term floating input correctly, then I would need for there to be a loose or open end connection somewhere along the circuit. Do you mean that the piezo element itself is nautrally a floating sensor input based due to the way it physically operates?
I added the 1Mohm resistor in series between the piezo and the ground, I also tried this in paralell. Is this what you meant? I just read up on pull-up and pull-down resistors, but that wouldn't really explain why the voltage is going up instead of the current.