Found a little puddle of coolant on the tray under the expansion tank after an enthusiastic drive back from Summer Camp.
First depressing thought was coolant overpressure escaping via cap, as it was right under the short "overflow" pipe. Put pink UV dye in tank and pressure tester on the system with coolant warm, pumped it to 20psi and wandered off for an hour.
Found this- hairline crack exactly under the arrow moulding. Not a split and no great coolant loss, even after 170 miles of mixed A and B roads and motorways.

Would have been pretty hard to spot the crack under normal running with just coolant so worth a close look at your tank next time you're in there. Tank is the original, by the way

Mine had a crack too, can't remember exactly where but it's out in the garage so can have a look and see if it's the same place. I replaced it and found another advantage, a new one is more transparent so you can see how much coolant you have without taking the cap off and peering down the hole.

Seen that a few times on VAG cars. The VAG bottles go yellow/brown over time with staining from the coolant, so swapping in a new clear one is a common mod to tidy up the engine bay, and the last one i changed was covered in hairline cracks and crazing. I'm amazed it was holding coolant!

I also had a fairly decent coolant leak on my A4 after a similar split appeared in the plastic coolant elbow that comes off the back of the cylinder head. Almost completely invisible on the outside as it was along a moulding seam, but on the inside you could see a distinct black line.

Mine cracked in exactly the same place.......

That's interesting. The fracture line has the same length and profile. Even the dribbles follow the same path.
Definitely something to watch for.