I couldn't have been the only guy that saw him.
So during the mission "Predator" in which you begin by switching to Franklin as he's chasing the O'neil brothers in their jeep, you play as Michael in the Company Heli (accompanied by the amazing Trevor Phillips) with a sniper rifle equiped with a thermal scope that you use to track down the brothers amongst the plentiful deer throughout the forest after their crash. When zoomed completely in you can make out the shapes of the heat signature easily.
Well while I was zooming in on random heat signatures I stumbled across a humanoid shape beneath the the signatures I was currently looking at. I panned down to it and it was standing very still. There it was, Bigfoot. No doubt about it. It had clear markings. Its belly had a circular patch of heat signature that was lighter than the rest of it, thick chunky extremities and a huge neck. Must have been 150% of the height of the O'neil brothers signatures.
Here's the weird part! When I switched off thermal to get a better look, nothing was standing there... just grass... I switched the thermal back on and... nothing. This was in a matter of 2 seconds. On, off "what?" on "WHOA!"
...just like that... Did anyone else experience this?