Strange damp patch

[mblago]

I have recently noticed a small damp patch that has appeared on the wall at the top of our stairs, about the size of a 50p. The wall backs on to our bath and electric shower and the damp spot is about level with the top of the bath. The tiled wall look in good condition and the sealant around the bath also seems good.
I have bought a metal/cable detector and does not appear to be any embedded pipework, I'm at a loss what it could be. Would be grateful for any suggestions.Thanks
Mike