Bummer! Sometimes they're not easy to find. Hope that's NOT the case for you tomorrow.:tu
When I sold my previous house the City had enacted a requirement that the seller provide a certificate that the septic tank/system was working properly. The house was new enough that the property map filed with the county had the septic tank location depicted. But when the honey dipper guy dug where it was supposed to be, it wasn't there :eek:. He kept digging holes and eventually found it, thankfully.
Related to Bruno's question, it's a local ordinance where I live. Septic tank systems are still popular in the more rural areas, usually on lot sizes of greater than 1 acre. In the more urban areas homes must be connected to the municipal sewage treatment system... ie, no septic tanks allowed.