Neighbours of Kweku Adoboli had noticed him working much longer hours than normal in recent days, doubtless assuming that the smartly dressed trader was engrossed in one of the big deals that had propelled him into the upper echelons of one of the City's most illustrious banks.
The sheer scale of the transaction that the 31-year-old had been working on became painfully apparent at 3.30am yesterday, when officers from the City of London Police arrived at a building believed to be the bank's Finsbury Avenue offices and arrested him on suspicion of having committed the largest single fraud by an individual in the history of the Square Mile.
His father, a retired Ghanaian United Nations official named John Adoboli, spoke last night of his heartbreak at hearing the news. "Fraud is not our way of life. I brought them up to be God-fearing and to appreciate decency," he said.
After a six-year career at Swiss bank UBS which saw him rise impressively from a trainee to a director in equity trading, earning a six-figure salary along the way, Mr Adoboli was allegedly staring at a £1.3bn loss after carrying out unauthorised transactions.
It is unclear how long the keen amateur photographer had spent trying to make good any shortfall, but his most recent post on his Facebook page, written on 6 September, was an indication that things were not going well. It simply read: "Need a miracle."
Since joining UBS in March 2006 as a trainee investment adviser, Mr Adoboli had developed an expertise in complex financial instruments called exchange-traded funds (ETFs) and a little-known trick of the investment banking trade known as the Delta One business, which can make vast sums from tiny fluctuations in the value of an asset or product.
But like Jérôme Kerviel, the trader for Société Générale whose unauthorised trading in 2008 cost the bank €4.9bn (£4.3bn), Mr Adoboli had also acquired experience in the back office of his employer, working as an analyst to explore the minutiae of the trading niche for which he later became one of UBS's four "market makers".
If it is proved that he committed fraud, it is possible this deep knowledge of his bank's procedures allowed him to conceal any snowballing losses.
Although he enjoyed some of the trappings of a City lifestyle, including a 3,000sq ft apartment, friends insisted he is far from the stereotype of a financier and was more interested in discussing photography or cycling.
Mr Adoboli spent part of his childhood in the Middle East before attending £26,000-a-year Ackworth School near Pontefract, in West Yorkshire. Former classmates said they remembered a "thoroughly decent" teenager who was keen on sports and expressed an interest in becoming an athlete or chemical engineer. Kathryn Bell, the headteacher, said: "He was an able student who made a very positive contribution."
After graduating in e-commerce and digital business in 2003 from Nottingham University, he moved to London and eventually entered the City.
Until May this year, he lived in a £1,000-a-week flat on Brune Street, near Spitalfields, a 15-minute walk from his office, before moving to another luxury flat in nearby Stepney. His former landlord, Philip Octave, said yesterday he had not been the "tidiest of people" and had twice fallen behind with his rent but was a "good tenant" overall. Mr Octave said: "He was a very nice guy, very polite. I would not say that he was a party animal."
Yesterday, as an international media pack descended on Mr Adoboli's former home, onlookers gazed up at a sign showing the building had once served as a poor house. Destitution may be the least of his concerns.