The bird-crap "burnt" it's way through the paint, and ended up looking like a 50p-size blister. Just happened to be slap-bang in the middle of the bonnet too, where it was most noticeable.
I use the wife as the test of noticeability, as she has got to be the most unobservant person in the world. If she can notice it, everyone can. So i said there was a mark on the bonnet of my car, she went outside and came back in 30 secs later saying "aye, there isny half!!" so that was the cue to have it fixed. If she hadn't noticed, i would have left it.