I had this same issue with my Zyxel GS1900-24e managed switch. It did not appear in the DHCP leases list, yet it was working like a charm. But I did not know its IP and thus couldn't connect to its GUi for maintenance, and I did not like that. Just for the record, and it may help someone after me with the same issue, here's what I did:
I unplugged the cable between the pfSense router (in my case, an SG-1100) and the switch.
On my Mac I configured static IP 192.168.1.2 with subnet mask 255.255.255.0 and then connected to 192.168.1.1 and there it was: the html GUI page of the switch. If this does not work, reset the switch by pressing the tiny reset button at the front using a paperclip or something like it for some time.
Then I configured the switch to use DHCP (in Maintenance > System > IP > Mode: DHCP). When that was done I configured the LAN on my Mac to use DHCP again and plugged the cable between router and switch back in. After restarting the switch its IP appeared in the DHCP leases list on the Netgate SG-1100.