I hear a hissing sound coming from my master bathroom. It consists of a toilet and a shower. My house is on a slab.

I can hear the noise throughout the room, but also from the shower head (a bit).If I go to the first floor and put my ear against the wall underneath the master bathroom, I can hear a fainter version of the same hissing sound.

I shut all the faucets off and made sure none of the toilets were running. I sat outside watching the water meter. It is the type with an arrow and a counter. I waited for abut a minute and neither the arrow or the counter moved.

Yes the noise stops when the ball valve after the pressure regulator outside the house is turned off.

Considering the extreme cost of plumbing work. I'm getting a bit fearful of what it is.
I've seen other posts of this sort, but they all seem to be obvious leaks. My meter isn't spinning. My water bills have not spiked.

If you can't see any water anywhere, it is somewhere that isn't obvious. Check the toilet first (in the tank specifically- a fill valve could do this), and any outside hose faucets that may have been forgotten. You could shut off the water at the hot water tank as well to determine if the problem is in the hot line. If the hissing stops after the pressure regulator "Yes the noise stops when the ball valve after the pressure regulator outside the house is turned off" is shut down, try leaving it on and shutting off the main valve inside the house if you have one. As that is the first stop before water diverges through the house, you would know immediately if you have a water line busted under the slab if it is hissing and the house is off. While it could be expensive, you might be lucky if the line is in good shape overall. I worked for a company that used a backhoe to pull sections of water line through the ground under slabs by attaching a new line to the old and using the old pipe like a tow cable pulling the new line into place. It wasn't always possible, but they did it about 30% of the time.

You've probably got an underslab pinhole leak. The pipes act as an amplifier and transmit the sound. A water meter will not detect minute amounts of water leakage.

California is still warm could have snake pissed off.

quote; try leaving it on and shutting off the main valve inside the house if you have one

He doesn't. That outside valve IS the main valve for the house, and it is the ONLY one.

Around here every house has the main shutoff inside, but it could be climate differences. The frost line is 36" and finding the shutoff could be difficult under a few feet of snow in the winter.

Yeah I don't have a valve inside the house. The only one is at the curb and one just after the pressure regulator.

My neighbor is a handyman/plumber. He was here last night. He couldn't figure it out either. There is no sign of water anywhere. This is the only room where one hears it. If you put your ear to the wall right underneath that room, on the floor below, a much more faint version of the sound is heard. He's very skeptical of a leak. He said he's had a pinhole leak before that took 3 days to pin down, as the amount of water was so small. Even at that point, the water meter was obviously spinning. For me it's not.

You can hear it from the shower head and the shower valve.

Yes the noise stops when the ball valve after the pressure regulator outside the house is turned off.

The noise appears when the water is on, it stops when you cut the flow. what part of that doesn't sound like a leak?
Your water meter should be moving at least a small amount over 20 minutes. it may be slow enough you don't see it, or maybe you don't want to. I know I would hate to see it spin with the water off in the house.

If it's a small pin-hole leak, your water meter will not be spinning noticeably, even after a minute or so. What I would recommend is using a marker to mark where the water meter is, and then come back about 30 minutes to an hour later to see if the water meter has moved at all. Make sure anything that would use water in the house is shut off. With what you're describing I would bet that you have a pin-hole leak somewhere near that room.

I, too, hear a hissing sound in the closet, which is adjacent to the bathroom. There is water leaking on the wall, where brick and slab meet, outside the house right where the bathroom is. I monitored the water meter for 8 hours and it did not move at all: leak indicator is not spinning, the sweep hand and the odometer read the same for the period of 8 hours. Since there is no movement on the water meter, does that mean we don't have a leak? And if we don't then where can the water be coming from? Help!