Yeah, I have had a few experiences with lost trackables before, for example...
TB6J07P was missing at GC3MKZW, until DavisDwellers found it at GCNBME, and it has been in my inventory on two separate occasions now.
When I found GC4862D, TB5PH2T was supposed to be in there, but it wasn't there! Apparently it's "been there" for three years now!
TB5JFPT was missing at GC4VPTG, until kapparos family found it, and then it was immediately lost again until 2RottenPears found it at GCYK16, and now I have it in my inventory.
So yeah, trackables get lost a lot. And as long as there are people who forget to log their trackables, there always will be.