In my new home I’m about to upgrade the thermostat. The unit is Honeywell AC plus gas furnace 2 in 1. New thermostat is a Honeywell as well. There’s no C wire in the wall, so instructions say: on the unit relocate the G to C and jump Y to G.
The unit’s in the attic. Upon accessing the unit I found the weirdest thing: there’s an extra wire bunch that connects to Y and C and then goes to a completely diff location than a thermostat wire pack.
It goes to the fireplace area. There I found a standard wall light switch next to the fireplace that is turned on but doesn’t control any lights. 
What in tarnation is this?
Extra photo for clarification, it’s really odd. I checked and the switch doesn’t control any lights or wall outlets.
Update. Looks like it has to do with the gas fireplace I apparently also have. I used multimeter to test the mystery wires continuity and they ring no matter the position of the mystery switch. So I guess maybe wall switch is unrelated. Anyway I just followed the thermostat instructions and left the mystery wires in place. Seems to work fine! Cools and heats. Lost the fan on demand function (G wire) but the instructions did mention that would be the case.
Update 2: Ok well I’m no Hvac tech that’s for sure 🥹 The “mystery” wires are just the outside AC compressor wires. It just so happens that the compressor sits outside the house behind the fireplace wall 🤦
As to the fireplace (I removed front panel and took a peek underneath) it was apparently gutted of all electrical components by the previous owner. The wall switch doesn’t do anything. It’s now just a “dumb gas”. There’s a physical gas valve next to the fireplace that one have to open while and at the same time trying to manually ignite the thing. Needless to say the whole disposition seems insafe and I probably will use it as a no-gas fireplace anyway 😂
