What was found
AFTER TAKEOFF A STRONG UNDESCRIBABLE ODOR IN COCKPIT, ODOR WAS NOT PRESENT DURING PREFLIGHT WITH APU RUNNING & BLEED AIR ON (APU WAS ON FOR 30 MIN) DID NOT ODOR LIKE ELECTRICAL, FUEL, OIL OR ENVIRONMENTAL ODOR PROBLEMS. DID NOTICE A FLICKER OF LIGHT ON THE MASTER CAUTION PANEL JUST BEFORE ODOR IN COCKPIT. DO NOT KNOW WHICH LIGHT, BUT IT WAS NEAR THE OIL FILTER BYPASS LIGHT. ODOR PROBLEMS. REMOVED APU OIL LEVEL SWITCH, CLEANED AND REINSTALLED. SERVICED APU OIL LEVEL AND OPS CHECK GOOD. REF 560XL MM 12-11-03, 49-10-00 (RE 100XL MM). REMOVED AND CLEANED COALESCER SOCK. REINSTALLED REF 560XL MM 21-51-00. OPS AND LEAK CHECK GOOD. NO FOUL SMELL NOTED.