Thomas and the Tide Mark

I'm always puzzled by the scene in 'Gone Gone' where Alison finds Thomas in the (empty) bath tub. There is what appears to be a tide mark around the inside of the tub but it either continues over, or can be seen through, the sleeve of his shirt. How can this be? Anyone know?

EDIT: I've been asked to add a spoiler alert tag. Seems a little heavy-handed for what is a very minor point, but OK.