Jet/Long Ranger/206 pilot/Inspector reported a bolt was missing from the tail rotor gearbox cover and was overlooked on inspection prior to flight.

Narrative

During routine maintenance. I was required to inspect work completed by the base mechanic on Aircraft X. Part of the work required removal of the tail rotor gearbox cover. Part of my inspection was to insure the installation of the cover. I checked to see cover and the bolts where installed. My inspection found no issues. Maintenance was signed off and aircraft returned to service. I later received a flight request that I launched on. We had a momentary delay after start; due to the crew could not talk on Intercommunications System (ICS). After a few minutes; they fixed the issue (ICS cables where not plugged in). We departed. Five Minutes into flight; we were canceled. On way back to base; decided to look at a control burn that was causing lots of smoke and visibility issues in the area. We flew a total of a .4 hours. We then landed back at the base with no issues. The following morning; I was informed by a coworker that they had found a bolt while cleaning the hangar and discovered that it was from the bottom side of the tail rotor gear box cover. They told me that they submitted a safety report about the mechanic; since they said they have been having numerous issues with his work. I informed them I may be at fault also since I could of possibly have missed it on my inspection; but I was sure I had seen all hardware was installed. I do not have a high confidence in the mechanic at this base. I have no proof but from interactions with him and other's stories of him; I would not put it past him that he may have known it wasn't installed. Possible solutions: When conducting inspections; I will run my hands over the bolts and physically get a count on the bolts. I will also include in my future preflight inspections to check for physical security of said bolts.