I examined the bike - and sure enough, there was nothing there at all. I then got back on the road and found that the same feeling could be achieved by going back on the yellow lines. What does this tell me?
Either that yellow lines have magical properties. Or they're deliberately concocted to feel a bit "slippery" and weird, so when cars and bikes go on them, they quickly want to get off. From what I can see looking around the web it seems that the paint they use can be sort of rubbery, so that may account for the odd sensation.
Speed bumps too?
This is now going to sound a bit nerdy ("when doesn't he?" I hear you cry) but I also wondered whether you can slightly benefit from this paint syndrome when you go over speed bumps, which can be terrible things especially if you're going at a fairly decent clip.
Here's the thing - I've noticed that if you aim your wheel so it goes over a painted part of the speed bump, you feel less of an impact than if you go for the other sections. Of course many bumps don't have the paint, so it doesn't apply. But try it - and let me know if it works for you.