Discolouration of pads on a motherboard

[smiba]

Hey everyone,

Every now and then I receive boards like in the pictures I've attached, what causes this discolouration?

The board does not seem to have any apparent water damage (no corroded pads or other signs) and the board is too dusty to be cleaned or even ultrasoniced by a different store.
It also aren't specific areas which are heavily affected, just certain (power)lines?

Thanks!

 

[dukefawks]

All normal tarnishing of the PCB plating, but in 2nd picture right in the middle there is a pad with corrosion, pretty obvious.