Went out to get in the car this morning and had a minor heart attack!
Around the inner circumference of the left front wheel is a fine coating of oil that lines up with the brake rotor!
I automatically assumed it was brake fluid that has leaked between the two mating surfaces of the caliper, and been flung around inside the rim. Pulled wheel off, doesn't smell like brake fluid; washed wheel, no paint damage.
Checked caliper, no apparent leaks, but two fine lines up the back of the caliper of oily liquid that line up with the inner edges of the rotor venting. Hmmm, WTF?
Worked it out; it's cutting fluid from when the rotors were slotted by the brake specialist!
They had a light coating on the rotor faces that I wiped down with wax & grease remover before fitting; but obviously some has run into the venting, and been flung out during the drive home...