Anyone ever seen this before?

This is located in the garage. It is a Honeywell product and looks like a thermocouple and an electrical cable to it, but they disappear into the wall. The exterior side of that wall has a metal cover, but nothing comes out from the cover. I have not seen this before and the house was built in 1956. My only thought is that this was for an old oil tank, because there is evidence that there was a boiler in the home in the past, but has been upgraded to a cat 1 furnace. There was no fill pipe or valve located anywhere

It’s not a thermocouple - they produce electricity to keep a gas valve open. I would say it’s a mercury filled thermostat, operating a SP switch. I usually see these used as lockout switches, i.e., to keep a boiler from turning on unless the outdoor air temp is 60 or lower.

Was/is there a boiler in the house?

As stated in the original post there is evidence of a previous boiler that has been removed and a cat 1 furnace is installed. I figured it wasn’t a thermocouple, but it looks like one. I am thinking your right though on the identification.

Here’s an OAT sensor from a commercial building located on a rooftop.

Is that an electrical cable or a capillary tube?

Temp sensor me thinks.

It’s a cap tube.