I said, "Huh?" as I'd not even been in that bathroom since we got home. Upon investigation, Lisa discovered a leak, and not where you'd expect. Rather than a leak at a joint, a tiny hole has opened on the line to the pressure-expansion tank. A fine mist of water was spraying from it. Lisa shut off the water input and electrical power to the hot water heater (there's a separate circuit and switch right next to the tank, per electrical code), and opened the taps to relieve pressure, and the leak stopped. But of course now we have no hot water.
We've concluded that while being without hot water for a couple of days is going to be annoying, it's also not a top-level crisis, so we're not going to call our plumber's emergency number. However, we'll be calling them first thing Monday morning and asking Steve-the-plumber to come out and replace that pipe at the earliest non-emergency opportunity.
Lisa put a fan blowing on the dampened carpet in the bathroom to try and dry it out. It's unclear how long the tank was venting, other than it wasn't doing so at 10 AM today before we left for Reno. We're hoping the amount of water that leaked isn't doing more than dampen the carpet.