And again, Bill, there could be extraterrestrials on our planet with isotopic ratios indistinguishable from our own.
Scenario 1 is this 2007 sci-fi short story by Greg Egan:
https://outofthiseos.typepad.com/blog/files/GregEganGlory.pdf
A tiny probe is accelerated to near lightspeed and aimed at a distant star system. Upon impact, the even tinier remnant of the probe "unpacks" and assembles sentient creatures and machinery using in-situ resources.
The tech to accomplish something like it is many millenniae beyond our own, if it is even feasible at all. However, I don't see how it would violate known laws of physics.
Scenario 2 is they arrived generations ago and have been reproducing while taking nourishment from local resources. Again, they are indistinguishable isotopically from us.
That said, I estimate the odds
against extraterrestrials (biological or machine) capable of self-directed action being here, now, as being super-cali-fragilistic-exponential -- much, much more than merely "astronomical".
I sympathize with the people who say "I want to believe". I am a lifelong sci-fi fan, love space operas, but at the end of the day sci-fi is entertainment-plus (the "plus" being the creative thinking it may spur and the countless people who have been inspired to choose science and/or technology as careers).
Of course my estimate may change (even flip) once I am shown hard evidence. I cannot list an exhaustive catalog of the forms that this hard evidence may take, because of the unknown unknowns. I will know it when I see it.