Form Populating with Wrong Information

This is a strange one... two different sales people reported today that when they click on a link to our email subscription landing page, the form pre-populates with someone elses's email, name, phone, etc. They each see a different lead's information. What could be causing this, and most importantly how do I know this isn't happening to external users? I confirmed that they have never filled out a form on these leads' behalf.  (This is a MKTO form and LP in case that's not clear.)

Thanks!
Susan