Anonymous lead information is based on the users IP Address pulled by the Marketo Munchkin Code that you added to your website. Any lead that has not filled out a Marketo form on your site, or clicked a link in one of your emails sent via Marketo will be considered “anonymous” until they complete one of these actions.
Additionally, Marketo’s Automatic Data Cleanup notes the following:
“Anonymous leads that have had no web activity in the past 90 days are automatically removed by Marketo. These anonymous leads are unlikely to return to your site with the same cookie. The visitor is likely to have switched browsers or cleared their cookies; you may already have the same visitor as a new, active anonymous lead or known lead.”
We have also found that if an anonymous lead has their computer set to always have cookies turned off, every time they hit the site, they count as another anonymous lead. We're facing a similar issue in seeing our anonymous list continuously increase at a much higher rate than l'd like to see. We're diligently working to provide reasons for them to somehow connect with relevant content so that we can get more of them tagged.
Munchkin.js sends activities to Marketo only when cookies are turned on. Your anonymous leads could be from Private/Incognito browsing mode or actual new anonymous leads.
Thanks Jessica for explaining how anonymous leads are acquired and converted/merged to known leads.
are there best practices for cleaning up/deleting anonymous leads to keep it relevant?
I'm wondering this same thing. I found this thread that was helpful: Deleting anonymous leads
It also mentioned that Marketo doesn't charge for anonymous leads and that Marketo automatically deletes anonymous leads older than 90 days. With this information, I think our policy is going to be to let the anonymous leads be and let Marketo handle the old ones.