for like 5 minutes tonight, I couldn't stop thinking about when this happened to me when I was younger and so I decided to finally google it bc I've thought about it a few times this year too. My dad, my sister, and I all lived in one house, we only had one house cleaning lady and no visitors ever besides her. We first noticed the crosses when the light hit them a certain way, they were at eye level on all the doors to our bedrooms and bathrooms. All of us and our cleaning lady could not figure out where it came from, so we figured it's a ghost or something, but we were the first people to own the house. The only thing I could think of is that it was my grandmother who had just passed away. I'm not sure what it was, but it's really cool to see this happened to other people.