I agree with you about the data centers, that's why some sellers report sales coming in clusters with zero sales in between and that some of the clusters all come from one area, (say) N.Wales.

In some off my small Saved Searches I've found an item of interest, not saved it and the next day can't find it again.  But a day or two after it re-appears on the Sold page.

As you say ebay will always find some results for a buyer's search, but I'm never sure I'm seeing everything that's listed.

As an aside and going slightly off-topic, I think it's all those data centers spread all over the world that makes sellers reluctant to trust ebay with their NINO, probably with good reason.