This morning, I heard the same sound of water running behind the upstairs shower wall and the tub spigot still had a slight drip (about 1 per 3 seconds). The water meter showed water usage, as expected. I went through a series of shutting off the tank supply, opening faucets throughout the house, turning the supply on and off, etc. There was a sudden rush of air and rusty water, and then everything was back to pre-problem normal! No sound of running water, no drip at the Moen tub spigot, and the meter is no longer moving. If there was a leak in the slab pipes, it got plugged by the rust. The sound of water movement was always loudest around the 2nd floor shower/tub walls, so some mysteries go unsolved. I never turned the water on to flush out the Moen guts while the cartridge was out, so maybe there was something there distorting one of the rubber pads or O-rings? The valve works better/easier now than it has since 1985 when we built the house. Hope it stays that way. The power of prayer!
Thanks to all for the wisdom -- I am a lot smarter now. John