@theskelly
If available, enable and configure DHCP snooping. Don't forget to enable the static entries to bypass (generally prefer to trust a MAC over a port, in case something else ends up on the port). This combined with ip arp inspection can significantly reduce noise and coerce clients to operate correctly. If that network exists anywhere in your environment, you might have something acting as a logical bridge between VLANs.
You might opt to end the rule set for the interface as (especially for 'untrusted' segments):
pass from <interface network> to <appropriate destination/s>
block log from any to any
May seem a bit strict but ensures that only accepted sources transit the firewall's interface, inbound. Making sure that any earlier rules don't allow bypass.