The data of 4,545 TalkTalk customers were found online through a Google search by researchers at the BBC.
The investigation by BBC Watchdog found that TalkTalk failed to inform these customers that their personal information was stolen as part of the 2015 data breach.
The information taken included bank account details, names, addresses, dates of birth, and mobile numbers.
The initial breach saw the information of almost 157,000 accessed, with over 15,000 customers having their bank details taken as well.
The investigation found that the information for 4,545 customers were accessible through a Google search, now four years on from the initial data breach.
These personal details are suspected to have been online since the breach, without the knowledge of the people affected.
A spokesperson for TalkTalk said: "The customer data referred to by BBC Watchdog relates to the historical October 2015 data breach. It is not a new incident.
"The 2015 incident impacted 4% of TalkTalk customers and at the time, we wrote to all those impacted.
"In addition, we wrote to our entire base to inform them about the breach, advise them about the risk of scam calls and offer free credit monitoring to protect against fraud.
"A recent investigation has shown that 4,545 customers may have received the wrong notification regarding this incident. This was a genuine error and we have since written to all those impacted to apologise. 99.9% of customers received the correct notification in 2015.
"On their own, none of the details accessed in the 2015 incident could lead to any direct financial loss."
When the breach happened in 2015, the Information Commissioner's Office (ICO) investigated and found a series of faults and failures in TalkTalk's security.
The ICO handed TalkTalk a record fine of £400,000 to reflect "the seriousness of the event."
