Sometimes you can get a leak in a pipe jack boot on your roof, so water runs down the outside of the pipe from the roof, then at the first bend or junction, the water leaves the pipe and settles on whatever horizontal surface is available. That's just an example of how you can have water in an area that doesn't otherwise have pressurized water lines near by, but may not apply to your situation. Also, if the toilet is up against the exterior wall, there could be a leak around the wax ring, but under the floor, that allows just a bit of water to seep out everytime the toilet is flushed. If that's the case, it could run along the top side of that flat panel in the photo. Hopefully that's not the case though.