Continuity mistake: When Ben is driving the car away from the store where they buy the comping supplies, the headrest in the car is missing, but it reappears in the next scene.
Continuity mistake: When Gina is sitting down with the camera at the beach, she picks the camera up from the towel, doesn't move an inch, but in the shot from above, the towel is nowhere to be seen.
Answer: They're not afraid the other couple are the killers, they're afraid they may be recognized on the video and that the other couple might figure out who they really are.