"Problem with not going into the floor is that you do not know how much damage is under neath, soil missing etc. "

This is very true. One good thing is that we have a code that any in slab pipes have to be sleeved. More than likely if it's just a small leak the sleeve contained it so there would be little to no soil erosion. If it's really blasting out of there it will cut through the sleeve and leave a nice big void under your slab. Do you hear water running? Another indication would be that your water heater ( assuming that it's gas) would be constantly burning to keep the water heated. If you have a stethoscope you can listen to the pipes too.
Matter of fact there is a company called " American Leak Detection" They will charge you a minimum of $350 to come out but they have sonic listening devices etc. They can tell you exactly where the leak is. The last time two times that I used them they found a guys long lost sprinkler valve under the oak tree roots, and on another one they found a slab leak in the middle of a lady's bedroom 18" under the slab.

 

Yes, you definately have a leak in the hot pipe under the slab. it's not uncommon. I have used American Leak Detection for years to find leaks in or under slabs too. Don't worry about errosion. That's not a problem. If the water heater can still keep up with the leak and produce hot water (instead of only warm) it means the leak is still slow and there should be no damage of that kind. No further proof is needed, but if you want more evidence, go out to the water heater in the morning before running any hot water and follow the hot pipe with your hand. The hot pipe will be hot a long way from the water heater, and the cold pipe will be cold right to the heater. Also, the water heater will be on more and more untill finally it never shuts off and can't keep the water hot. The noise is water running.

If you have radiant heating in your slab let me know before digging to fix the water pipe!

Wetspirit

 

I forgot to mention one thing. Some homes have type M copper under the slab. Sometimes it can be leaking in one spot but be so weak that the fix is not practical or is short term. If you can, look into bypassing the leak instead of diging it up. Especially if you have thin pipe, corrosive soil, or a radiant slab.

Wetspirit