I'm pretty curious how they advertise the fact that they do something like this as well.
Some time ago on HN I remember reading about a company that embeds forms on websites. So if I filled out a contact form on Site A, the third party collects the information, stores a cookie on my computer. Then when I visit Site B, the cookie uniquely identifies me, and the third party company gives my email address to Site B, even though I didn't fill out a form.
Is this what you're suggesting marketo does?