What brand (and model, if you can tell) single-handle faucets are in the house? Not all of them can fail and allow a cross-over, but some can.
If you have an under-slab leak, this time of year, it's easier to find than in the summer...if you have a cat or dog, they'll probably gravitate to that spot because it will be warm! When there is no water use, check your water meter. It might be moving slowly, so if you can avoid water use for an hour or more, go back and check to see if it has moved. If it has, you might want to start checking around for things that are leaking...could be a toilet, or the hot water supply line. If you've not used hot water for awhile, if you feel the outlet of the WH, and go away along the outlet run...it should be colder as you go away from the WH. IF it is hot for a long distance, you either didn't wait long enough, or there's some flow. You can then try turning off the inlet water, and let it sit...if that then allows the outlet to cool, you know there's a hot water leak somewhere.
In your bare feet, you can often tell where an under-slab, hot water leak is. It would likely be easy to find with an IR camera, and the cost of them has come way down in the last few years...you can get a neat one that plugs into a cellphone or tablet for a bit over $200 now. To get that performance a few years ago, it was in the thousands.