Only once did I ever see anything in the sky I couldn't explain, but I'll never forget it.
Way back in July of 1982, I was camped out in an alpine meadow at about the 6,000 foot level on Mount Rainier, Washington.
My friend and I were just sitting there next to a small fire, when I spotted what resembled a very bright star moving across the sky from east to west. It was moving at a steady rate, and I estimated its altitude to be more than twenty miles. It didn't look like much, just a light that could have been a satellite or something.
We watched the thing move steadily across the sky. Then...without any hesitation it made a ninety-degree turn to the north. The turn was absolutely instantaneous. It continued at the same rate of speed north, until it eventually disappeared over the horizon. We agreed that no man-made object could possibly make a turn like that...but, I still have no idea what it was.

That sounds pretty much unexplained I've seen similar once and while it's easy or obvious to think it's just a satalite or shooting star, deep down you know that explanation doesn't seem right.

Well, it's difficult to gauge the altitude of a pinpoint of light when you're standing on the ground.
That said, it could have been at a much lower altitude, say 40,000 to 50,000 feet.
Back then, there were U2s and SR71s in the air all the time. They are not used so much today, owing to the proliferation and improvement of satellite surveillance.
But the 90-degree turn is a puzzler, isn't it? It may be attributed to the angle at which you watched it.
Who knows?
By the actual definition of UFO it qualifies: Unidentified Flying Object.
I was born and raised in Washington state. More than once, as a kid, I saw strange lights overhead in Spokane. We attributed them to B52s or KC135s from nearby Fairchild Air Force Base. Now I'm inclined to believe I may have seen a U2 or SR-71.
I saw a U2 up close at Howard Air Force Base, Panama, in the late 1970s. It was white. Until then, I'd thought all U2s were black. I have no idea why it was white, but such a color would greatly reflect sunlight.
Perhaps what you saw was, indeed, an alien craft getting a scenic look at magnificent Mt. Rainier. Pity they didn't use their ray gun on a few Californians as well!

Yes...it was the absolutely instantaneous turn that made our mouths drop open like a couple of idiots. I knew right away that no man made object could do that. I am sure it was above 50,000 feet, but who cares, right? It was all about the turn.