Yes tar, fall out, tree sap, general rubbish, a claying will make the paint smooth as glass to the touch of your finger tips, also try the crisp packet test, using a foil type crisp packet split in half and rub against a clean car and you'll probably find it catches on the paint, clay bar it and then try the test again

Also see

https://www.two-sixties.co.uk/mgoc26...ead.php?t=4723