Adoboli, 35, was released from Maidstone Prison in southeast England last week, having served half of his sentence, the source close to the investigation said.
The Ghanaian-born banker was convicted to seven years for fraud by abuse of position for losing the Swiss bank's money, and to four years for a second count of the same offence, which he was to serve concurrently.
He was sentenced in November 2012 but had already been detained for nine months prior to his trial.
During Adoboli's trial, he claimed senior managers were fully aware of his activities and encouraged him to take risks to make profits for Switzerland's biggest bank.
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But prosecutors said that in a bid to boost his bonuses and chances of promotion, Adoboli exceeded his trading limits, failed to hedge trades and faked records to cover his tracks between 2008 and 2011.
The tactics initially paid off, but as the financial crisis took hold, Adoboli's deals went bad.
The court was told that at one point the privately-educated son of a former United Nations official was at risk of causing UBS losses of USD 12 billion.
Adoboli's arrest in September 2011 wiped 10 per cent of UBS's share price.
He had sent an email in which he confessed that UBS was exposed to colossal losses resulting from his unauthorised trades.
During his trial, the court heard that Adoboli began conducting off-the-books trades in 2008, holding them off the ledger until the market rebounded.
However, he racked up huge losses and discrepancies in his trading activities eventually aroused suspicions.