So far, here's what I've got:

1. Contact water company
2. Contact plumber
3. Check water bills
4. Check water meter against sprinkler system
5. Check for a warm slab
6. Check with city to see if house was built over some sort of water
source
7. Check with neighbor on water bill (elevated?)
8. Dig exploratory hole (just deep enough to investigate).

******* Any other suggestions out there? *********

Thanks.

Scott