On card details pages, under the viewcount where users are listed as circles, how are they chosen?

I'm trying to prune unused cards. I'm an admin but don't think I have access to the activity log, I've seen it mentioned but the option doesn't exist anywhere.

Some cards have hundreds of views, but the only avatar I see is my own. On other cards I see other people. Does this mean no one else has viewed it recently? Is there a cutoff, like after 30 or 90 days the users fall off that list? Is there any other way to determine the last viewed date of a card?

I looked at the domostats connector report and it shows me created and last modified date but not last used.

Thanks.