I'm assuming they are using Lead Forensics - I've heard of this solution before. There are a few other competing technologies, but LF is probably the most well known.

I think they do IP identification from a large number of network sites.

You fill out a form w/ company X and they're sharing data with LeadForensics, now when you visit company Y, if LeadForensics got your IP from company X, they know you visited.