OK, so the increased temp coming back in boosts internal pressure? That to my thinking means that, once subjected to cold temps, a gearbox's internal pressure's decreased, maybe causing a small amount of air to be sucked past its seals, yes? It's THAT additional air that expands once warmed, leading to potential for oil leak... no matter how temporary.

My approach would be either to leave the bike in the cold when not in use, or instead plan on opening the oil fill port when brought inside to equalize the pressure as it returns to room temp.

I'd also want to explore the question with a techy of whether a lower viscosity gear lube would be of advantage when used at low temps outside. Standard stuff may become rather thickish, making pedaling less efficient and maybe also not doing the best job possible of reducing friction in bearings and on gear teeth. It's not like you're gonna be pedaling furiously along enough that friction alone's gonna warm that gear case much....

(BTW I'd like to see your '15 Tarmac Disc sometime here. I bought a '23 model late in '24, love it when I can ride it outside!!)