I did that once on a VA MEL-SYD flight.
Boarded late (after lounge boarding calls were made) and found a rather large bag under the seat in front of me. What ensued was a LOTFAP style panic of that bag being "unattended baggage". Fortunately it was in J of which the FAs were much more attentive!
Ha. Could you imagine that in LOTFAP? Fox News would cut into regular programming with a "possible terror attack at airport"; the national threat level would go up; the plane would be towed to a remote stand and passengers denied dismebarkation for hours; sniffer dogs sent on board . . .