White stain on tile grout - sign of water leakage?

[CountryB]

I am getting a white stain (maybe efflorescence) on the tile grout in my bath. I don't see any obvious signs of water leakage (looking in the opening behind/under the toilet it all looks dry, and the wall paper and wood trim are dry). 

What do you think is going on?

Has anyone had this in their coach?

[Dr4Film]

Is it removable like on the surface (powder like)  or is it "stained" into the grout?

[CountryB]

It can be scraped off, but not brushed/wiped off.  I'd say it is more of a "stain".

I have not tried to mess with it yet - but what I did mess with seems to be in top grout layer.

I can uses a grout saw (like toothbrush sized) and lightly scrape to a clean layer of grout. So this white stuff is on the surface. I suppose that means it is not coming "up" from underneath due to wet plywood. Maybe water sat "on the top" of tile (window left open in rain, or water spilled) although I cant recall either of those things happening.