Also Strugatsky's "Roadside Picnic" - the "picnic" in question was a visit of aliens on Earth for unknown reasons. They came, they went, no one has actually seen them but they left their trash on our planet, "artifacts" in zones that are cordoned off by governments of Earth - from infinite batteries to multiplying gels and various gravity fields that will rip you apart in a second.. Like you said, some people ascribe all kinds of intentions to this visit, but most likely it's an encounter with a bug for them - they just left their stuff without as much as noticing us at all.