Isolation Transformer low voltage question

I modified  an isolation transformer, but still have some low voltage AC between secondary lines and outlet ground, even though grounds are not connected.   See attached.  When I measure AC voltage, with a DMM, between outlet ground and left L1 or right L2 outlet inputs I read ~6 volts AC on left and ~12 volts AC on right.  Outlets are plastic and not grounded to case, so I'm not sure how I am even completing a circuit.  I measured resistance between outlet ground and left, then right inputs and this reads infinite resistance (open) which makes sense.  So I am unclear how I can be measuring some low level AC voltage between grounds that are NOT connected and the isolated L1 and L2 lines.  Hope to hear back from someone smarter than me!   THANK YOU.