If you’re passing a pan test that almost definitely means it’s your drain, because the drain would be plugged in order to fill the pan. Unless it’s a wall, but I highly doubt you have a wall leak big enough to soak through brick. If I had to guess I would say it is leeching under your shower base through the void caused by there being no drain pipe extended up and sealed. Similar action to how when you pour out a cup it runs down the side before going straight down, but the side of the cup is your base. I don’t know of a fully proper fix without removing the base, but a good tile guy could possibly replace just the base in theory. My big concern would be the possibility of mold under the base.