I have a suspected drain leak inside a commercial wall with multiple fixtures. Can see no active leak but will get standing water between walls. Other than removing a twelve foot section of wall is there a more user friendly way to go about troubleshooting? We've discussed removing and remounting fixtures one by one but at best this is shooting in the dark. Would dye be an option?

Fill some buckets with water and save for later. Shut down your main to see if it is actually a supply line causing the issue. If you can dry between the walls the floor should stay dry as long as you keep the main shut down, this would inicate a supply leak and not a drain. Put food cloring in one bucket and dump into a fixture, see if tha color water appears on the floor. If no water then try a different food color in a different fixture and so on.

Not sure what your is but you have mold between the walls by now which could create issues with the health department and employees so some part of that wall wil have to be opened.