Clicked link via email server spam protection?

We are beginning to leads that appear to respond to email, by clicking links, but we are seeing no other activity (2300 leads)

Since this isn't a small number of leads, we begin to wonder if there is a new form of spam filter that might be testing links in a safe environment before allowing mail to be delivered to the recipient's in box.  Could this be possible?  All these leads are part of a set of historical records we pushed into a new nurture stream, so they are regularly getting mail.  It just doesn't make sense someone click large numbers of email and not ever fill out a form, download an asset, visit a webpage -- or anything else.

Anyone else seeing this kind of thing?

Could there be a new form of spam protection that is clicking through links in a sandbox?

Other thoughts?

Thanks,

Shari