Inaccurate Web Activity?

It is likely that the one of your rep's _mkto_trk cookies for your domain became incorrectly updated to point to that unresponsive person's lead record.  This can happen if the rep clicks a tracked link in a sales insight email that they originally sent to the person (e.g. in the email in their Sent Items folder or in a reply that includes the original email). 

You can determine if this is the case by having the rep go to any of your form's that pre-populate the email address and see if the address is theirs or someone elses.  If it is someone elses, they need to delete the _mkto_trk cookie from their browser.

Also, from your description it could just be a link/trust checker running on their email client or server, which will generate hits with tracking as it goes through emails looking for virus/malware/spoofed links to flag.

Thanks, Elliott. That was the first possibility I checked, and the rep confirmed that they hadn't clicked any links in their Sent Mail. I ran your test just to confirm, and it looks like that's not the issue.

Craig, I hadn't thought of that, but that's an interesting possibility. I suppose there's no way to check for that?

There *should* be, but I'm not certain, haven't gone looking.  Would be something like looking at the browser criteria of a click, but that's only available for 'visited web page' and 'click link', not on any of the email filters like clicked link in email.  And since the 'bot' doing the clicking almost assuredly doesn't accept cookies, I would guess that it doesn't get past the tracking link server and on to the actual page result, but maybe...  and then if you got something that far, what that browser agent string might look like is anything under the rainbow, pretty much...  here's a couple of link checker user agent lists
http://www.user-agents.org/  infosyssec.com/infosyssec/security/useragentstrings.shtml  just to give you an idea of what you're up against on this topic...

Do you see that pattern of activity for any other leads at that company?  If not, I'd be surprised if it was something on their email server checking the email links.  Plus, why would it check the links in one email every day?  Did you get a delivery or bounce notification?  Its also possible that the recipient forwarded it to other persons, which might account for an increased number of visits - how many clicks were associated with the email?

There are a few emails, but a couple of them have 10+ clicks associated. Seems like it could be a forwarding issue.