You're totally right about the flashlight making water drops stand out! @the_dylan has a point about angled light showing every little detail. I keep a small LED one in my toolbox just for this kind of thing. It's way easier to move a light around than trying to position a mirror perfectly. Both tricks are smart, but light wins for speed.
Wow, that's a brilliant trick with the mirror! It's amazing how such a simple tool can save so much hassle. Spotting leaks early really stops small issues from blowing up. I've had times where missing something like that turned into a huge repair. Using what you have on hand shows you know how to fix things. Glad you caught it before things got worse!
Actually, gotta agree with blakepark here about the flashlight. The mirror trick @the_elliot mentioned is clever for sure, but shining a light across a surface shows tiny beads of water way better in my experience. A mirror just gives you a flat reflection, but angled light makes every little drop stand out. It's saved me from missing a slow drip more than once. Still, any method that finds the leak is a win lol.