Tracking issues

Hi, we're seeing a spike in leads that their original referrer is a page on our website (for most it's the homepage but there is a substantial amount of leads with a specific blog post being their original referrer). 

While it makes sense for some of the leads to see an ad our ours or hear about us somewhere and go directly to our homepage, it doesn't make sense for the spiking amount we're seeing and it doesn't make sense that someone will go directly to a specific blog post on our website where the URL is more complicated and specific. 

We are using cookies with hidden UTM fields for all our paid campaigns on all of our forms and it's working fine. I'm asking about all the leads that did not come through paid campaigns but went directly to our website with no other referrer in the way. 

Did anyone encountered something like this? a more than reasonable amount of leads that come directly or without any trace to their website? Is there something we can do that we're not doing to reduce this number? 

Thank you.