In Francis International Airport during Free Mode, the player can sometimes see a pedestrian walking around the runways. He looks a bit like Little Jacob with his second outfit. If the player attacks him, even with explosives, he won't die. He will stand there for a few seconds before turning and shooting the player with an MP5 until they die. The only known way to kill him is by knocking him into the water and drowning him. It is believed that he is likely just a glitch.
This is a proven phenomenon, but it occurs quite rarely. It is unknown what factors of the game cause this glitch to occur. It is possible that there are some old unused pedestrian paths that run through the airport, but they were not removed from the final version of the game, and the peds spawn by accident in multiplayer.