He will disappear after a few seconds, but that heat signature belongs to none other than Bigfoot.
When looking at the O'Neil brothers' heat signatures in the woods, aim for the body at the lower right. GTA 5 is, by far, the biggest game in the franchise, with a massive world map and complex story that takes dozens of hours to complete. However, during the mission Predator, you'll take control of Michael while he snipes from out of a helicopter.
Well, back to GTA: San Andreas at least and while Franklin can encounter a 'bigfoot' in the Strangers & Freaks mission The Last One, it's clearly just a guy in a suit.
