The very small chance of it being aliens is the same as the very small chance of it being wizards, or angels, or the rogue appendages of extradimensional horrors, or glitches in the matrix. It's irrational to fixate on the technically non-zero odds of aliens when there are an infinite number of explanations with technically non-zero odds.
Our Galaxy formed 13,000 million years ago. We estimate any single technological civilization that developed the technology to build Bracewell/von Neumann probes during this time (perhaps minus a few billion years) could have colonized every star system in the galaxy with self-replicating tech in at most a few million years. Presumably if the probes were self-replicating they could also be self-maintaining and therefore a single colonization could effectively produce a permanent technological presence within a given star system. We can't know what a civilization would do with such a network, but we can guess that human beings would program it to investigate life and technological civilizations.
So the "could there be alien technology in our solar system" question breaks down to a series of technical questions around the feasibility and probabilities of the above events. I personally still think those probabilities are low enough that we can round them to zero, but the spec-fi fan in me very much hopes that they're not.
Also: for god's sake, these are Chinese balloons and not aliens. :)
I'm guessing, but is it because we'd see a big empty space with a large gravitional pull? Would light bend around it similar to a black hole, or is the behaviour different enough to be obvious?
Disclaimer, my knowledge extends to high school physics (which I've largely forgotten).
In an "infinite worlds, infinite possibilities" sense it's possible that something, somewhere in the universe is alive and/or magical. But there is no telling that 1) that's actually happening, and 2) that we'd ever encounter it before the heat death of the universe.