Never seen anything but I think its certainly possible. Statiscally they just about have to exist. But in order for them to travel here they would have to be able to manipulate the laws of physics, time,space,gravity. If you can manipulate one, you can manipulate them all, which is actually a pretty scary thought. We're actually getting pretty close to. The main issue is it takes an enormous amount of energy to do so which we don't have now. I would hope that if we do ever get there all countries sign some sort of an agreement that time will never be manipulated.