yeah, the media is full of chemistry experts...

if it's something that shows up when the skin of the plane gets hot (like when the APU is powered up), then it could be something like LEMON JUICE (remember the old invisible ink as a kid?)
I think that was a reaction with the paper tho- so probably wouldn't work on a painted surface...

but you get the idea- just something that changes color when it gets above a certain temperature.