Gas fireplace is connected to the thermostat. Is there a way to disconnect?
I have run into this a couple of times.  If your situation is the same as mine, here is what's happening:  The fireplace operates on extremely low voltage. Because of this, it doesn't require much outside influence to induce a voltage in the stat wire to open the gas valve on the fireplace. If when the fireplace was installed, the installers ran the stat wire through the same drilled holes with line voltage circuitry wires, this is how it happens. Someone turns on a chandelier of the refrigerator starts. Those circuit wires experience a significant up tick in current flow.  That current induces an induction field all around the cable. If the stat wire falls within that field, the fireplace starts. It is like a mini transformer.