It's not a byte sequence, it's a BREAK == pulling the line low for longer than a byte, i.e. invalid serial framing on the stop bit.
I assume the possibility of spurious Rx state is why FTDI chose to wire Rx to the tip pin of the female TRS connector: It's the last pin to make contact with anything when plugging in, and therefore least likely to be bridged to ground. I suppose the mechanical design of any particular jack would determine whether it's possible at all.