Opening at Adrian's request.
If an ath(4) card doesn't get its initial power on config from EEPROM, the driver doesn't attach. This is usually some power up reset bug in how the BIOS sets up the PCI bus. The PC Engines APU appears to do this at times, seemingly only with an older BIOS rev that was buggy in that regard.
from Adrian: "I'll go see if I can at least have the driver explain why that's an invalid id at boot up"