How to track if a specific entity has been seen by the player

Just as the title says. What I'm looking to achieve is the effect of a mob spawning in nearby due to an unrelated event but if the entity is "seen" by the player (and what I mean is the players crosshair crosses over the entity's model) I want the mob to start a chase sequence with the player right after. I've been trying to research how to get this done but have ultimately come up without anything to show. Does anyone here have any input on how to get this working?