Shower – Mustee 3060L shower 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...)

I have never set a shower pan in a mud base, in glue, yes. Whirlpool tubs go in a mud base with building paper first, reinforcing wire on top of that, nailed or stapled in place, then a stiff layer of high strength sand mix cement or mortar piled high and the tub "settled into" the mud so the bottom of the tub carries the weight, not the edges.

If you did all that with your shower pan so the mud base did not crack, it would not need glue under it. Typically shower pans do not have that much room under them to allow enough mud under them to keep it from dying out too quickly and therefore weakening the mix, that is why glue has been the only thing used that I know of.

In my opinion, without knowing more of the situation, as in not able to hear the type of sound it is making, you will need to pop the pan and reset it. I think the mud base may have been your own idea?