This morning I woke up to see water leaking through the drywall ceiling of the kitchen. I drilled a few holes and got all water to drain off.

Then I went upstairs and everything looked fine in the bathrooms. Then I heard a faint dripping sound in the master bathroom within the wall behind the shower knob. The shower knob was slightly open but no water coming from the shower faucet. I pushed the faucet knob and the dripping sound stopped.

My question is if someone moves the knob to this odd setting again, it may leak again. Do I need to replace the cartridge?

Plumber
Will create a plumbing access
Test for pan leaks
Spray kill whatever mildew/worse that now lives there
Repair water damage
Fix leak
Properly caulk/seal problem areas
Guaranty their work
THE END of it for hopefully 20+ years

I would replace it

Shouldn't be related to the setting of the valve. Should not leak in the wall period. Only possible exception would be if you could see the low flow coming out of the spout or head and running back into the wall somehow. But that's doubtful.

Is pretty weird if you are not getting any leakage from normal usage. Gonna need to open things up and find the exact source.

Did you confirm it doesn't leak when opened in any setting? I know you shut it off and stopped the leak. But if you open it up all the way does it leak or no?

This happened to us about 6 months ago. Decided we wanted to remodel the master bath anyway. When contractor ripped everything out, he noted that the pony wall was installed with the incorrect material. Luckily we caught it before any mold/mildew took root but had them spray it down anyway just in case. This is what you get with newer houses nowadays I guess.