Wow, that's quite the story! Situations like that can be really eerie and unsettling. When faced with strange or unexplained situations, it's good to approach them logically before jumping to conclusions.
1. Reflection or Light: Sometimes, windows can play tricks on us. It could've been a reflection from a nearby source, a passing car, or even a play of shadows.
2. Animal: Occasionally, animals like a cat or a bird might cast a shadow or be mistaken for something else, especially in dim light.
3. Robber: If it was an intruder, there should've been signs of a break-in, or they might have left in a hurry upon realizing people were returning. However, if nothing was missing and there were no signs of forced entry, this seems less likely.
4. Paranormal: While many people are fascinated by ghost stories and supernatural explanations, it's always good to rule out all the logical explanations first.
Given that nothing was taken, and there were no signs of anyone being inside, my bet would be on some optical illusion or play of light. Still, always good to stay cautious and maybe even install some security cameras just for peace of mind. How did your dad and his friend react afterwards? They must've been quite shaken up!