We actually have a lot of data about UFOs.

If they are spacecraft, for instance, they are a very dirty and destructive way to travel.

Even on this slim point, you can reason backward and have some insight into the civilization that created them.

If these spacecraft are armed, then the weapons they use are strangely primitive. No, not bows-and-arrows primitive. Just strangely unsuited. Like they were never designed for serious war at all.