I have a bathroom room on a first floor slab... the toilet overflowed and water is now seeping under the tile to the hallway and getting the carpet wet... we turned the water off behind the toilet... and it seems to stop leaking... we don't see and water coming out around the base of the toilet.. where is the leak coming from?

When the toilet overflowed, the water ran under the tile, under the sill plate and is seeping into the hall. Unless you have a crack in the toilet, which apparently you did not have before overflow, the water should dry out. Wet vac up as much as possible and get fans blowing in the hall. You also mentioned that the water stopped leaking when you turned off the stop valve. Get the toilet blockage cleared and then, after everything dries out, turn on the water and look for any leaks.

In addition to overflowing the back up may also let water out under the toilet at the wax ring. I'd consider going a bit further with the remediation since this was sewer water and the likelihood of mold.mildew is great. Can you detach the carpet and roll it back so it dries from both sides? Get a wet vac and a dehumidifer too along with Mold Spray Bottle 946mL Concrobium Mold Control Product | Concrobium Mold Control (http://www.concrobium.com/whereToBuy_profiles_cmc946ml.php) . If the water stopped leaking when you closed the valve we may have more than one issue here. The normal overflow is a back up due to a clog. The fill valve would have nothing to do with that.