BigWallop wrote:

How do you mean " run up hill" ? The water could be puddling under the
shower tray, and when it reaches a certain level, or gets to an easy flow
out point, it will drip through. So it could be your waste pipe.

The point from which the water drips is higher than the plug hole. The
waste pipe, by necessity runs downwards. Water isn't pooling inside the
shower, and gravity prevents it from pooling on the underside of the basin.

Consider the shower basin to be this shape:

--x--\ /-----
\________________y___/

Where Y is the plug hole, and X is the approximate point of dripping.

Drew