Is this singe-handle shower valve servicable? If so how would I find the source of a steady drip when the valve is shut off?
Glad you found your answer.

I've also had slow leaks (drips) like you experienced due to (1) solder spatter inside the valve that prevented a good seal on the rubber, and (2) plumber soldering the pipe with the cartridge in place which hardened the rubber so it didn't seal well.