The water comes from the plastic tube that comes out of the box?

It is usually connected to the pressure release device (PRD).
It may just be leaking.
It may be you have another leak or condensation inside the unit which is flowing on the outside of the pipe.
It may mean that the input pressure to the shower is too high.
It may mean that you have a fault in the shower causing the water to overheat, giving a pressure rise.

Engineer
February 2010