looks like you are in for a long fight.
if you tape the shower head shut, and you tell the builder and his plumber that you are about to do this, it means you are ensuring beyond all doubt that no-one is showering from that shower, even when you are not home. They will understand that they are dealing with someone who is methodical, able to plan a process of elimination, and able to communicate and explain it in advance. This is the best kind of correspondent to be if "they" are honest, and the worst kind of adversary if "they" are dishonest. They may be operating with all the good faith in the world but just not be able to see the leak. It happens.
Either way you win. Eliminate that possibility.
diagnosing plumbing problems means getting down on your knees or belly a lot and getting dirty often. You now have to do it too since they are not coming back until the problem presents more information that will enable an accurate diagnosis. Enjoy the stretching, take it easy, like yoga.
the first time around it is normal that you wouldn't learn all about the "thingie" that had to be repaired... now you have to get into this subject and study what there is there, what connects to what, what was apparently diagnosed, what was changed, what might be leaking, and more.
put a few plastic bags over the shower head and tape each bag shut. Layers of plastic; look very convincing.
post more here about the distances involved.