Leaking Mixing Valve
Here we have a video of a leaking mixing valve in a shower. The valve is old even though the shower has just been retiled. The flippers did not bother to update the plumbing while they had it exposed.
The valve is obviously leaking out of the front, but this sort of valve relies on o-rings to maintain the seal. It could also be leaking out of the back as well, depending on how many o-rings are worn out. If it is leaking out behind that trim pieces or the back of the valve, water is getting into the wall cavity behind the shower. Water in a nice warm cavity with lots of wood will grow fungus and molds. All that pretty new tile may have to come off to repair the damage.
The good news is that most leaky shower valves are repairable. A good handyman or plumber should have the unit repaired so that water only comes out where it is supposed to.