I've posted before about these stains on the slab parge coating that I'm getting around my entire house. It was concluded that moisture was wicking up the parge coating when it was raining or damp outside. However, I noticed this one area pictured below that gets darker in the morning. There is a bathroom with a shower on this wall, and we've had a guest using it. I'm wondering if this is what you'd see on the outside of the slab if maybe the shower drain was leaking?

Afternoon pic:


Morning pic:

I can see your concerns and basis but
when looking at a well watered lawn close neighbor & assuming city water with its salts,
why not include efflorescence as a suspect as well?