Technician reported landing gear pins were not removed after routine maintenance; resulting in an air turnback due to the landing gear not retracting after takeoff.

Narrative

On Date; [a] lockout tagout was performed for a flap change. Gear pin installation was required for lockout. A coworker installed the gear pins and I documented all lockout tagout for flap change. [On] Date 2; lockout for flap change needed to be reversed. My coworker removed the gear pins. I witnessed him returning gear pins to where they go in the cockpit. I did not verify how many he had. I looked at main gear and nose gear to verify no flags remained. I did not see flags on main or nose gear. I signed off reversal of lockout for flap change; which included gear pins being removed and signed off.[On] Date 3; I was notified of an air turnback for Aircraft X for MLG not retracting. I notified my coworker of this and he said he was not aware there were 5 gear pins on the 767 and he only removed 3. He said he did not see flags for other gear pins also. The gear pins were missed by the Mechanic who departed the aircraft as well as the pilot who did the walkaround. My assumption is storms in the area [on] Date 1 possibly blew gear pin flags to a position where they were not visible.