Dropped the undertray and found a drip of fluid on the bottom of the power steering cooling pipe, at the point where it goes under the radiator.
I think it was just engine oil, as there was a slight stain on the radiator directly above (and I did have a spillage 2 oil changes ago).
Working hypothesis is that the extreme heat in the garage over the past week made the oil really runny and finally make its way out.
I’ll keep an eye on it and see if anything reappears.