A hijack threat to a Mumbai-bound Air India flight before take-off on Wednesday morning that prompted security officials to park the aircraft in an isolation bay of the airport here and initiate statutory checks turned out to be a hoax, an official said.Authorities at the airport said flight AI 2772 departed at 5.38 pm after necessary checks as per protocol. The flight was originally scheduled to depart around 10.30 am. The note threatening that the plane would be hijacked at 11.10 am was found in the washroom just before take-off, said Vikram Singh, the Director of the Kolkata Airport.All 105 passengers in the aircraft were deboarded and it was taken to the isolation bay, he said.