Just all things that have to be eliminated.
It looks more and more like a seam in the roof or the junction between roof and wall. Running water on it until it leaks again seems drastic, but may be the only way to fine the entry point.
I spent a long time looking for a leak at the roof line of a bedroom slide and finally found that it was getting in behind a trim strip about a foot above the floor level. It would run down and out at the floor, but also wick up and appear at screw heads halfway up the walls. We were trying repeatedly to seal the roof seams until one night when there was a tarp over the top of the slide and rain started coming in anyway.
Water was sheeting down the outside wall, getting behind the trim strip and following the screws into the wall cavity.
Elastomeric paint solved the problem.
Good luck with yours. Finding the leak is the hard part. Sealing it up is far easier.