I've now captured this twice, in two different builds. I'm assuming it's a temp-related bug because I can't make any logical sense out of it (or I'm just missing something).
Mechanical airlocks will heat themselves up within seconds of being built, even in freezing temps and with no other heat source around them (and without power connected). Then they start heating up the surrounding area.
P.S. I just realized these pics are from the same build. Thought I took a pic from my latest build, in which the same happened, but I guess not
Find a cold place, and build a mechanical airlock.