A PLT ON A NIGHT CHKOUT FLT WITH AN INSTRUCTOR INADVERTENTLY ENTERS CLASS C AIRSPACE.

Narrative

I WAS DOING A NIGHTTIME CHKOUT FOR PROFICIENCY AND FAMILIARIZATION WITH THE AREA. WE DEPARTED HARVEY (S43); DID THE REQUIRED MANEUVERS AND FLEW TO 75S USING PILOTAGE (EG; FOLLOWING INTERSTATE 5 AND VARIOUS INLETS). AFTER DINNER AND A POTTY BREAK AT SKAGIT BAY; WE DID ANOTHER SET OF MANEUVERS IN THE AREA; THEN DEPARTED FOR HARVEY. DURING CLBOUT IN THE PATTERN; WE HEARD TFC INBOUND AND THE LOCATION HE WAS BROADCASTING WAS FAIRLY CLOSE TO US; AND WE TRIED LOOKING FOR THE TFC. IN DOING A FEW S-TURNS AND LOOKING/COMMUNICATING; WE FOUND THE TFC WAS NOT WHERE WE EXPECTED; EG; FURTHER OUT. UNFORTUNATELY; NEITHER WERE WE; AND IN THE PROCESS DRIFTED INTO NWU CLASS C AIRSPACE. WHEN WE REALIZED THIS; WE CLBED AND EXITED PROMPTLY. CONTRIBUTING FACTORS: UNFAMILIARITY WITH THE AREA AND PILOTAGE AS PRIMARY NAV (NDB WAS INOP; PAE VOR WAS NOT BEING RECEIVED WELL). CONCERN ABOUT INCOMING TFC. I BELIEVE WE SHOULD HAVE STAYED IN THE PATTERN AND BROADCAST OUR POS MORE VIGILANTLY -- IN ADDITION TO XWIND; DOWNWIND; INDICATE END OF RWY; PARALLEL TO; ETC.