Pesky Leak that Can't Be Found in Old Tub/Shower

I have a house built in 1940 - plaster walls, tile floors, etc. Evidence from a leak appeared in my entrance foyer ceiling a while ago, and we have been trying to find the leak ever since. Plumbers say it is the drain on the tub/shower overhead, yet there is NO leak if just the bathtub is run! The leak only occurs when the shower is on! They want $5000 to rip out the ceiling and repair the leak! I have been investigating on my own and have heard that if there is a leak in the plumbing (mixing valve, shower head) that occurs when the shower is on, that it could be "spraying" and causing a drip that occurs below but not exactly directly under the source of the leak.

Has anyone had experience with this type of pesky leak? Looking at the plumbing is difficult due to hardwood and tile floors and plaster an inch thick. I am now removing the plaster from around the shower arm so I can investigate that area, next will be expanding the hole in the closet that is behind the faucet plumbing and seeing what is there - I don't see any leaking around the mixing valve area.

Any help in finding the exact source of the leak will be most appreciated.

Hold off on tearing into the ceiling. Although it will probably have to be repaired eventually, and it is true that opening it up may provide visual location of the actual leak, start by investigating the shower. Remove the little chrome eschtcheon around the shower arm ,and look in there with a flashlight. Remove the trim plate from the shower valve, and look back there with a flashlight, and lastly, if the shower is turned on by pulling up a button on the tub spout, then remove the spout and see if ther is evidence of a leak on that connection, going back into the wall. It can have a leak which does not occur during a bath, but only during a shower when pressure is increased at that tub connection.

Pesky Leak

Thank you - I am going to check behind the shower arm tonight and will check those other areas - I'll post what I find, hopefully I will find the location of this leak!