For anyone else with this problem, here are two things I tried:
I changed the PCIe slots on the motherboard!
I removed the Coral driver plugin, restarted UNRAID, installed the Coral driver plugin, then restarted UNRAID again.
Before I had my Ableconn PEX-MP117 adapter card in a different slot at first and I was geting the "No EdgeTPU detected" error. Today I tried the above things and I now get "TPU found" instead.
Ableconn card notes:
The card has jumpers to change between "PCIe based" and "USB Based" mode. I left mine in the default "PCIe based" position (jumpers on 1-2 & 4-5).
I only plugged the Coral mini PCIe board into the Ableconn card. I did not connect anything to the USB port on the back, nor the J5 header on the board.
I'm not sure exactly what worked but, this detectors config is working for me now:
detectors:
coral:
type: edgetpu
device: pci