BFI used to be the Ameriflight base and would be moved to Spokane during peak season due to the enormous amount of cargo just for Seattle area taking up all of the ramp space. During that time the BFI-BLI run still flew out of BFI. Now the Spokane is the Ameriflight base I would imagine the aircraft seen at BFI is either that BFI-BLI run or a schedule from the PDX base.

I haven’t worked there in over a decade, but I wouldn’t recommend it.