A Reaper courier only appears in the series of side missions "Spy Games," in which Cole MacGrath must follow him to a specific location. The speed and sight range of the courier changes between missions.
Depending on which mission of "Spy Games" is being played, a courier may be able to spot Cole even from up high if he peeks off the edge. He carries a flashlight with him, which makes it easier for Cole to spot him. Every now and then, the courier stops and spin around a bit to see if anyone's following him. On the ground, these are the times he will most likely spot Cole. When a courier reaches his destination, he deposits a package, which contains Blast Shards. Cole cannot kill the courier until they deliver their package.
If a courier spots Cole, he takes out his bombs, growls and charges, making them similar to the Reaper Mad Bombers. If Cole is on higher ground, he stays below him and eventually explodes, which causes the mission to fail.