From what you have said it sounds like it is the vapour from it venting to atmosphere, which is really what you want. From the factory this vapour would be piped into the inlet tract and so eventually let out through the exhaust, but this is bad for performance! On the rally car we just plumb a pipe out the wheel arch, on road cars you can run a pipe down the tunnel past the passenger compartment. I have seen touring cars with the catch can just inside the rear bumper but thats a lot of pipe work. Basically its a case of piping the outlet some where you are happy with. Oh yeah and some tanks have a breathable cap too, in some cases this can be done away with but check first.