The gray wire sheath looks similar to something I found on my E30+. There is some kind of sensor mounted in the front of the stove cabinet just above the cabin sole. On the face of the cabinet there is a small, round fitting with a metal screen in the center. Behind the cabinet, which I can't access to see, there is some kind of fixture that feels like it may be similar to what you have in the picture. The gray wire runs into the engine compartment, but has been disconnected. (Don't really know if the engine compartment is where the wire is supposed to terminate since it has been disconnected.)
I figured it was some kind of sensor for propane gas that would automatically shut off the gas line if there was a leak. Or maybe, as mentioned, it is a heat sensor that would do the same thing in the event of a fire.