It is possible for meteorites that were never found. Those that were found have been already ruled out as spacecraft debris. I mean, it has to be obvious that something is not coming from a naturally occurring piece of rock.
For those that were not found, it depends, how many spaceships do you think are out there? We certainly have a lot of rocket stages debris in the oceans, even Mir debris, we haven't found most of them as far as I know, so you might have be some extraterrestrial spacecraft debris from the dawn of time lying somewhere too. As far as direct physical evidence such as debris goes, we have nothing to increase or decrease the probability of alien spacecraft visitations. We have not found any to increase it, and we haven't expected any to decrease it.
But the more fun question is - are there any terrestrial spacecraft from the dawn of time? I find those particularly appealing.