Return current in a pcb stripline with floating plane
Even thos the floating plane has no DC connection to the other plane, there will be parallel_plate capacitance between the two planes.
C = Eo * Er * Area/Distance
C = 9e-12 farad/meter * 4.7 * 10cm * 10cm / [1/16th inch / 3]
where we assume 4 layers of copper foil, and 3 layers of FR-4 insulation
C = 45e-12 farad/meter * 0.1 meter * 0.1 meter /  0.0005 meters
C = 45e-12 farad/meter * 0.01 meter^2 / 0.0005 meters
C = 45e-12 farad/meter * 20 meters = 900 picoFarads
and at 1GigaHertz (given 1pF has reactance of -j159 ohms at 1GHz)
our 900 pF has reactance of 0.18 ohms.
But that is a distributed reactance, and 10cm is a large fraction of the wavelength of 1GHz.
Summary: there will be useful coupling between the floating Ground and the non-floating Ground.