They show up on radar, have supposedly shut off and/or activated nuclear weapons systems, IIRC, here and in the former USSR.
That is a very old story about messing with the nuke systems and I don’t think the gov has ever confirmed that. Nor was it ever connected to sightings like they are admitting to now IIRC.
Yes, they show up on radar, then disappear without moving and reappear in the same place or a different place. They do all kinds of things on radar that no object should be able to do. But that isn’t threatening in and of itself. They can’t even say that they are objects.