It's honestly more likely that these (as in, everything we've seen as a species that we currently would potentially consider a UFO) are black bag programs with a terrestrial origin than they are from another planet.
If we're going to entertain the option that it's extra terrestrial in origin I think we have to consider one of two likely scenarios. First that it's from a species a far enough distance away that FTL propulsion is required for them to reach earth. The issue here is that they would have such an incredible mastery of physics that any technology they developed would be (if they chose it to be) unseen, unheard and entirely unnoticed. It would be as much more advanced than we are, as we are of some our brighter non human inhabitants on the planet. They would have long known that we were here, and sentient by the shroud of electro magnetic communication signals we've been sending out for about 100 years. If they were choosing to not be seen, they have a funny way of going about it. If they're choosing to be seen, they again have a funny way of going about it. If they're ambivalent about being seen, why bother spending the resources on showing up in the first place. Earth isn't a particularly interesting planet beyond the fact that it's the only spot in this particular cosmic neighbourhood that has obvious sentient life.
If they're from the immediate cosmic neighbourhood (where FTL isn't required to send mechnical probes...we're only a generation or two away from sending stuff to neighbouring stars) we probably see evidence of them before we see little probes buzzing F18's
This is all ape brain conjecture from someone with a limited understanding of even our limited understanding of the universe and what is possible technologically within it, but I think occams razor applies here. The simplest explanation is that there is off the books tech we just haven't seen yet. There's a pile of examples of this phenomena through out modern human history. I've followed tech news since I was a teenager and there's been a lot of theoretical breakthroughs over that time that you don't hear so much about after it occurs. There's good reasons for most of that, not economically feasible, science journalism being shit in general and pumping up everything as a new breakthrough to generate readership/clicks, etc but I have a hard time believing that we don't have some crazy black bag shit when they black bag skunkworks that we know about (Darpa) has done this (that we know of) over the last bunch of decades:
1969 - The internet
1969 - the first generation of GUI that would later go on to become Windows
1971 - Street level mapping (beat google maps by almost 40 years)
2003 - Voice recognition AI (Siri, etc)
1973 - GPS
1970 - Stealth technology
2002 - the underpinning technology of autonomous robots (cars, the little *******s that run around the Amazon warehouse picking orders, etc)
and the new shit that we know about?
2014 - Excaliber laser. Can hit targets in space and is 10% the size of current high powered lasers being worked on commercially by the defence industry
2012 - Started studying the concept behind the film Avator.(anyone else notice the explosion of brain interface tech over the last couple of years? Limbs, vehicles, etc)
2018 - UHPC, apparently the protoype is out this year. 50x more power efficient than existing processors, with an exaflop of computing power. The Chinese are apparently playing with a 55 petaflop (55 quadrillion operations per second), and exaflop is 1000 petaflops....and is the same unit of measurement most researchers use to describe the amount of operations the human brain is capable of.
I'm pretty sure an unknown Darpa project that has been around for years could look a hell of a lot like aliens to us, and Darpa is just the program that we know exists.