Lookdown is meant to find only one byte (character), not a series. If you are only interested in whether or not "CB2E" is present in the incoming data stream, and you aren't concerned about where it is in the string, you should you the command

serin2,PORTC.4,16390,1000,NotFound,[WAIT ("CB2E")]

; Gets here if "C2BE" is found within 1000 msec of invoking the command

NotFound:
; Gets here if "C2BE" is NOT found within 1000 msec of invoking the command
;------------------------------------------------------------------------------------------

And one more thing: If your device has a hardware serial port, PLEASE use it.