There was Photographic Evidence !!!
The night of Sally's abduction, immediately before being caught by the three things with a rope, Sally tried to defend herself with a flashlight. Because she had been heavilly druged, she was unable to hold the flashlight.
When the three 'things' caught Sally and were dragging her across the floor, she had presence of mind to grab a camera and tried to defend herself with the camera flash. We see, in two of the flashes, some of the best shots of at least two of the creatures we ever see in the whole movie. Sally actually took pictures of the 'things' that abducted her !!!
Of course, the camera flash didn't last long enough to really ward the things off. They went right on with their plans.
Nonetheless, I wonder what they thought when they found the camera and developed the film ???