Things You've Noticed After Many Viewings.

After destroying the compound, Billy is walking away still laughing at Hawkin's joke when he suddenly turns around like he sensed something was there. In actuality, he heard the Predator. If you listen closely, you can hear the Predator mimic Billy's laugh. Billy heard it but wasn't sure if it was an echo (which was on his mind thanks to Hawkins) or something else.

The landmine Mac sets up to trap the Predator is the same one he and Blaine disabled at the compound.

The rattle sound that Hawkins hears right before being killed is not the Predator. It was actually a rattle snake. Snakes can see in infrared, and it probably saw the Predator and could sense that it was a threat. We are shown a snake in a tree just prior to Anna running, and while it might not have been the same one, most likely there was another nearby.