You don't have to assume FTL. They could be extremely long lived, or not alive at all (AI). They might be from within our solar system. It's also not valid to

You don't have to assume FTL. They could be extremely long lived, or not alive at all (AI). They might be from within our solar system. It's also not valid to reason "because I can't guess their motive they can't have one.". Using humans as an analog people travel to go to the zoo, for religious reasons, or dispatch probes and rovers. UFOs might be any of these or others for aliens.