Shower Plumbing or Concrete pan
I think the fact that a "dark damp patch appears on the concrete" when you open the shower valve(s) yet catch the water in a bucket points directly to a leak from the plumbing. You need to open the wall from behind to gain a view of the shower valve body, and also remove all trim on the inside. Use the handles to open the valves and find the leak source. Water may be running down the interior of the enclosure wall. The fact that you have a plumbing leak does not mean that your shower pan is in good shape; if it is old, since you have removed the tile, this is an appropriate time to break out the concrete and have the pan refurbished (hot-mop, vinyl liner, fiberglass pan, etc...)