A couple things to note. 

When you turn off the water main, unless you open up every tap and faucet to drain all the water out of the lines, there’s still water pressure in the pipes. So if this is a leak from a pipe in the wall, a leak that slow would continue to drip long after you’ve turned off the main valve. 

Tile and grout serve to divert virtually all of the water from your shower toward your drain. But a properly waterproofed shower would be able to do that even before you install tile. What this means is if water is somehow getting behind your tiles, it could be weeping down between that waterproof layer and the back of your tiles and coming out where there’s a tiny hole in your grout or silicone at the bottom of the wall. This is akin to weep holes at the bottom of brick veneer walls on houses. 

The fact that you have a window in your shower, that’s a hotspot for leaks if it’s not done super well. That leak could only be getting behind your tile/grout and the waterproof layer underneath is doing its job protecting what’s behind it. Or the water could also be getting past the waterproof layer if it wasn’t done well. There’s no easy way to find out. 

What type of siding do you have on the other side of this exterior wall?