Do you mean the cargo tunnel?
A "cargo" tunnel did indeed exist on the other side of the airport. One day, the BOAC flight crew were taken from the BOAC HQ car park to the central area via this tunnel for some reason. The Capt. had his wife with him in the crew transport, as she was flying with him on a Staff Ticket on that occasion.
They were stopped at the entrance by a security officer speaking in an accent that could have put him from that large island to the West of England, the Southern part of which doesn't form part of the UK. He examined the crew identity passes, and then announced that Mrs. Capt. was not allowed through the tunnel, which was not available for passengers, staff and crew only.
No manner of identification from ticket, passport, Capt's declaration would persuade this guardian of LHR. that the lady should pass. Eventually, when tempers and commonsense were getting fully exhausted, this genius noted that the crew transport was holding up the then RailAir Link bus from Woking Station, full of passengers off a B.R. Train. Tell you what, Sir, he said to the Capt. your wife can get out of the car, on to the bus, and on the other side she can get off the bus and into the car again.
That happened, LHR Security is well secure in hands like that.