Mystery Water Use in a New Build Condo

Hello r/plumbing, hoping some of you have some ideas on how I should proceed with a plumbing issue.

Background: I am in a new build, multi-floor condo with no prior owners in California. The building has submetering for each unit.

When I first purchased the condo, prior to moving in, I received a water bill measuring approximately 1000 gallons of water use, and has consistently showed 1000-1300 gallons of water use each month despite not yet moving in. My unit has shutoff valves for the main water line, another shutoff before my in-unit hot water heater. Additionally, I have an in-unit washer/dryer, dishwasher, shower/bath, faucets in the kitchen and bathroom. My toilet uses reclaimed water and is not measured by the meter.

There does not appear to have any clear leak in any of the floors, walls, faucets, toilet. No moisture noticed in the unit. There is a drip tray around the water heater which continues to be dry. No water in the dishwasher or washing machines.

The water lines, as far as I can tell, look like the following:

Main water into the unit // water shut off valve // submeter // split to various cold water uses and to the water heater.

Cold water // shut off valve // hot water heater // Everywhere else (2 faucets, dishwasher, shower, washing machine)

Meter does not run in the following conditions:

1.      Main water valve is off

2.      Main water valve is on, water heater valve is off

Meter runs in the following conditions

1.      Main water valve on, water heater valve is on. Everything is on

2.      Main water valve is on. Water heater valve is on. Water trap primer valve is off. Hot/cold is off for kitchen faucet, hot/cold is off for bathroom faucet. Dishwasher is off. (I am unable to reach behind the washing machine to turn off the valve and the shower given its in a wall)

When the meter is running, it is not a constant slow movement. I often do not see any movement but over a period of maybe 8 hours, can notice a difference around 10-15 gallons. I have spoken with the developer about this issue, who sent someone out to run a prong-free moisture meter on the floors, walls, checked the toilet, and looked around for any obvious leak without much success. They have just recommended I check in with the submetering company without doing further diagnostic tests. They did not test anything with my hot water line, only to suggest there was no leak, from their basic walkthrough/visual inspection.

Interestingly, the developer needed to fix my shower mixer valve because it was somehow inverted. During that fix, there was no clear moisture around the mixer when they replaced it. Also when I described my issue with a plumber inquiring about services, they suggested it might be unintentional water use by another unit given its a new build?? (not sure if that could be possible and if so, how would I even confirm that?)

Does anyone have thoughts on how I should proceed? I assume plumbing in this situation is covered under California’s SB800 but every time I ask the developer to look, not much is done. If I ask the metering company to check their meter, they tell me it could be $400 if there is no fault found within the meter itself. I can also look for an outside plumber to assess the problem as it seems like the developer is not doing anything, but that can cost $600 for their 2 hour check. I worry if I go through any of these actions and pay out of my own pocket, will not be reimbursed if something is ultimately found. In the meantime, I have turned off the water to the water heater but would appreciate any ideas to identify the issue or just how I can navigate this situation.