Battery positive under water, turned water red?

[VinceFox]

Now, i'm not bad at spannering but i know sod all about chemistry, electrics and so on. This afternoon ive taken a battery out of a car that was standing in water, up to a live terminal junction box, due to a leak in the roof.

My question is this: why was all the water in the boot rusty orange, when there's no rust anywhere on the car?

[littleredrooster]

A hidden red sock somewhere?

[Pandaboy]

Electrolysis?