AC-Current to ground without a closed circuit?
Continuously flowing current requires a closed loop.

Current, or rather charge can flow without a closed loop. However, it will not flow continuously because once the potentials equalize the movement of charge will stop. (Think static shocks and antennas which are often dead end conductors).

There is such a thing as common mode voltage and common mode noise. These are perhaps the terms of the things you are referring to.

Continuously flowing current is like pumping air through a pipe. But you can also pump air into dead end pipe or tank. Once the pressure equalizes, the movement of air stops.

That said, I think your example is not quite accurate though (I don't really understand what it is you are trying to say). It sounds like you might be confusing the voltage between the TWO terminals of C1 (not relative to GND) and the voltage of C2's top terminal relative to GND.