If our planet has existed for billions of years and has experienced extinction level events in the past, it is very possible that a previous species existed here very much like we do now facing death and maybe forced to leave.  It may be the natural cycle for Space.  When a species becomes technological enough they realize their own mortality and to ensure the continuance escape to Space.  It would not surprise me in the least if the ufo encounters popular throughout our planet are previous inhabitants checking out their old digs or new peeps checking out the neighbors.