I'm trying to remove a heat detector in our kitchen left by the previous owner to replace with a Nest protect.

It is hard wired with two red and two black wire connections. They're smaller than wires for switches/outlets so I'm assuming it is wired to alarm - do I need to cut power to the alarm box before I unwire it? I'm assuming I would just use wire nuts and connect red to red and black to black to remove it and then just cover over with (battery) Nest?

I loaned out my circuit tester to a buddy so I wanted to ask before I zap myself, haha.

I would leave it there. Heat detectors are in kitchens because kitchens of false alarms generated from cooking smoke. At best it will drive you nuts going off with every puff of smoke that come from crumbs in the toaster.

Sounds like a 2-wire system where you have red in, red out (interconnect) and the black (power). One of your breakers should control the power and as with all electrical tis recommended to turn off and not play with hot wires. As mentioned, smoke detector in a kitchen is a bad idea. Why not leave as is and just add a Nest in another area?

And it may be wired to alarm with red, but that is likely NOT where the power is coming from.