Even though we can’t know how much traffic you’re missing due to untagged campaigns (see here what I’m talking about: Why should you tag your campaigns for Google Analytics? ) here is a hint where this traffic is landing on the website.
We started from the premise that is unlikely that a user will type in the browser URL bar a link longer than the website domain. Based on the research we had performed in the past years we found out that traffic which landed on pages like www.domain.com/my-long-landingpage-name is often, in fact, traffic from social campaigns from platforms like Facebook, Twitter, LinkedIn. The campaign are usual not properly tagged.
Look at the list of landing pages and try to remember where you’ve used these links. You will most likely find campaigns that you’re running and you were wandering why your missing revenue for them in Google Analytics.