Seeing something that you can't identify does not mean it "must" be of alien origin.
I think astronomers, especially armatures (who tend to actually look at the sky) all have probably seen things we can't explain. Growing up in a very small town and probably the only person in town with a telescope, I use to get many calls from locals asking "what is that thing off to the <compass direction>". Sigh. Sometimes it is easy. "Oh, that is Venus to the west after sunset", or "Oh, that is Jupiter to the east after sunset", then to explain the concept of opposition. But there were a few that required more study.
Living in No. California (no, not SF, think almost to Oregon), we would sometimes see the results of rocket launches from Vandenburg in central CA. back in the 70s, there were a lot of experiments going on releasing ions high in the atmosphere. They still do them today. Well, they would launch north up the coast of CA and we could see them. Each time someone would eventually ask me, what was that in the sky the other night? We would also get sounding balloons too. I saw one that was brighter than Venus before sunset to the west. Turning my small refractor on it I could easily make out the inflated Mylar ball and the payload below it. I was able to follow it till well after dark.
another time I saw a light making strange turns in the sky to the east. Could tell how close it was since it was dark and I had now way to triangulate, but I knew there was an empty field past the trees in front of me so I walked down the hill to the road that lead to the field and hurried out to see if it was close. It was windy that night, masking any sound, so I had to get pretty close to figure out it was a small plane doing aerobatics at NIGHT. No idea who it was.
More recently there are the occasional Iridium or other satellite flares (which can be researched now with many software offerings), but to get to the point...I can only recall seeing one thing I couldn't explain.
Back in the early 70s, a local community college held an annual event to commemorate the death of JFK. Each year on the Thursday after Thanksgiving, there was a 50 mile (almost 2 marathons) walk/race that started around 9pm. They laid out a course though the backroads in the area and had it well organized with water/potty stops along the way and rides back to the college for those who wanted to quit early. Well, I was walking with a group when one asked "what is that"? we all looked over towards where they were pointing to the west and I saw a light (about as bright as a 1st mag star) slowly moving north to south (from our perspective) about 5 degrees before it switched direction and repeated. It was only a couple of degrees above the top of the hill it seemed to be over. I watched it for a good 5 minutes before it finally just dimmed out. My guess was it was some kind of search/rescue going on with a helicopter, but the hill was at least 6 or 7 miles away so here wasn't any sound. never did hear anything else about it and to this day I don't have a better explanation. Oh, BTW, I made it half way, mostly walking and my feet still haven't forgiven me!