"tripled"??? The theory below is not going to account for a triple unless you are a very light water user. Are you saying water use has tripled or even doubled? That's a bunch. But your leak detector shows it is not just an increase in water bill rates.
Was a check valve installed at the same time as the heater? That could keep the expanded warm water from feeding back into the pipe coming in. You may be losing water out of the T&P valve. You can put a pan under that outlet to see.
This theory would only account for some intermittent loss, and not big volumes. The cure could be to add an expansion tank after the check valve. The alternate cure would be to replace the faulty T&P valve that is stuck open.. but that would have been replaced when the water heater was replaced... so scratch that theory.
You could get a water pressure gauge that records the highest pressure seen. Put that on the laundry sink faucet (with hot tap on) or washing machine hot tap for a while. See if the pressure rises high.
http://www.homedepot.com/p/Watts-3-4-in-Plastic-Water-Pressure-Test-Gauge-DP-IWTG/100175467 has an extra "lazy hand" to remember the max pressure.
It could also be that the city changed your meter and added a check valve. Same cure.
"tripled"??? This theory is not going to account for a triple unless you are a very light water user. Are you saying water use has tripled or even doubled? That's a bunch. But your leak detector on your meter shows flow. This is an unusual set of symptoms that you have described.