C182 sufferd an NMAC with a Piper Arrow while practicing a search grid adjacent to an airport.

Narrative

We were on a practice search mission using a grid search pattern heading southwest at approximately 1200 FT AGL; when an aircraft passed overhead. From later conversations; it was a Piper Arrow that had just departed a nearby airport. We were monitoring 123.0; and were being flight followed by approach control. We heard no calls from the other aircraft; although the individual at the FBO states that the other aircraft made all the appropriate calls. We subsequently landed at the airport; making calls as appropriate in the pattern. No responses were heard from the FBO operator; however another aircraft departing our landing runway after we landed was heard calling his departure; and responded to our radio; so we were on the right frequency. A couple of comments: 123.0 is very busy in that area. Frequent simultaneous transmissions occur. It may have been that the calls from the Arrow were blocked by other calls in the area. It would be very helpful to provide more frequencies for CTAF calls.