A security guard dispatched to investigate an alarm at the Hatton Garden Safe Deposit was told to stand down as burglars operated below after a previous alert was caused by an insect, a court heard.
Approximately £14 million of jewels and precious metals were stolen in what is believed to be the largest burglary in British legal history when thieves drilled into the concrete vault after smashing an alarm.
The "highly sophisticated" system had alerted the vault's operators, put out of business by the break-in, who said they believed whoever was behind the burglary was party to "significant information" relating to the security features of the vault.
The perpetrators are said to have gained access via a lift shaft before leaving an "out of order" note on its door.
Four men, Daniel Jones, 58, Brian Reader, 76, John "Kenny" Collins, 75, and Terry Perkins, 67, have pleaded guilty to conspiring to conspiring to burgle the vault over the Easter Weekend this year.
Another four have denied any involvement in the raid.
Security guard Kelvin Stockwell agreed with a suggestion by barrister Nicholas Corsellis, defending Carl Wood, 58, who said the burglars would have required "detailed inside information" to be successful.
The men are said to have moved in after staff locked up for the Bank Holiday on the evening of April 2.
Mr Stockwell told the court how he was sent to check the building after the alarm was triggered in the early hours of the following morning, arriving at 1.15am to find no police in the area.
After finding the building's front doors and fire escape were secure he rang Alok Bavishi, a son of the building's owner, to say the building appeared "secure".
In a statement read to the court Mr Bavishi said he had received a call from a monitoring company to say the alarm had been triggered and the police alerted.
"They told me that the intruder alarm had been triggered and police were on the scene.
"I'm aware that this happened a few years ago and it was caused by an insect or similar."
After setting off to check on the building he received word from Mr Stockwell that the building was secure and thought it "likely to be a false alarm".
The court heard the burglars remained at the scene until around 8am that morning.
The break-in was discovered three days later when staff returned from the long weekend.
Mr Stockwell said: "I looked and there was a lock on the door and that had been popped, there was a hole through the wall and I saw that we had been burgled.
"On the floor there was drills, cutting material, the lights were on and the second floor (lift) barriers were left open."
He also found a note had been left on the lift saying "out of order" that was not there when he locked up, he said.
Mr Bavishi's brother, Manish, told the court his "theory" was the burglars had knowledge of the design of the vault, first installed in the 1940s, and basement as it "hasn't changed much".
The family had owned the Hatton Garden Safe Deposit for around seven years.
While they had improved the alarm system after taking the premises over the codes had not changed, he told the court.
He added that it was policy for security guards not to enter the building in the absence of police.
He added that the episode had left the company "insolvent".
Mr Corsellis said he made "no criticism" of his family or business for the huge burglary.
Wood, of Elderbeck Close, Cheshunt, Hertfordshire along with William Lincoln, 60, of Winkley Street, Bethnal Green, east London; and Jon Harbinson, 42, of Beresford Gardens, Benfleet, Essex, face the same charge of conspiracy to commit burglary between May 17 2014 and 7.30am on April 5 this year.
A fourth man, plumbing engineer Hugh Doyle, 48, of Riverside Gardens, Enfield, north London, is jointly charged with them on one count of conspiracy to conceal, convert or transfer criminal property between January 1 and May 19 this year.
They deny the charges. The trial was adjourned until Friday.
Mr Bavishi said his firm did not provide insurance for the safe deposit boxes, 73 of which were opened by the burglars.
The owner said that of the 999 deposit boxes in the vault, 562 were in use at the time, while 180 of the occupied boxes were on a "drilling list" as the users has not paid the rent for more than a year, the court heard.
As a result of the break-in the firm had been left insolvent and the company had been put into liquidation.
Mr Bavishi said he had provided police with the records of all his employees since his father took over running the vault seven years ago.
The family are not based in London but live in the Sudan, where he was at the time of the burglary, he told the court.
Two firms were used to supply and maintain an alarm system installed soon after the takeover, who would send contractors to service the system, he said.
In the event of the alarm being triggered he said there was a company "policy" that a member of staff would go to the building but not enter.
He said he did not know how to open the vault door himself and it was only the security guards who knew the combination.
The court heard that the lift had not accessed the basement level of the building where the vault was situated for more than three decades, after an intruder armed with a shotgun attempted to break in.