Congratulations to armadillogal and sparkyNM1955 for the FTF!
At approximately 7:40 pm the 727 took off with only Cooper, pilot Scott, flight attendant Mucklow, copilot Rataczak, and flight engineer H. E. Anderson aboard. Two F-106 fighter aircraft scrambled from nearby McChord Air Force Base followed behind the airliner, one above it and one below, out of Cooper's view. A Lockheed T-33 trainer, diverted from an unrelated Air National Guard mission, also shadowed the 727 until it ran low on fuel and turned back near the Oregon-California border.
After takeoff Cooper told Mucklow to join the rest of the crew in the cockpit and remain there with the door closed. As she complied, Mucklow observed Cooper tying something around his waist. At approximately 8:00 pm a warning light flashed in the cockpit, indicating that the aft airstair apparatus had been activated. The crew's offer of assistance via the aircraft's intercom system was curtly refused. The crew soon noticed a subjective change of air pressure, indicating that the aft door was open.
At approximately 8:13 pm the aircraft's tail section sustained a sudden upward movement, significant enough to require trimming to bring the plane back to level flight. At approximately 10:15 pm Scott and Rataczak landed the 727, with the aft airstair still deployed, at Reno Airport. FBI agents, state troopers, sheriff's deputies, and Reno police surrounded the jet, as it had not yet been determined with certainty that Cooper was no longer aboard; but an armed search quickly confirmed that he was gone.
Take a leap into the unknown to find the third clue. Mr. Cooper would advise you to look for an opening and search there. No bushwhacking is necessary to find this one. However, most end up missing the secret path, so you might end up going through the forest anyways.
Be cautious of muggles, as this seems to be a dumping ground for junk. Pull off the road and walk a short distance to this odd forest to find what you seek.