A family of three is said to have been recruited even though only one agreed to join the party in principle.
Labour earlier this month suspended the selection process in Falkirk to find a candidate to replace troubled MP Eric Joyce amid allegations of membership irregularities.
So far, the three contenders have been local councillor Linda Gow, communications expert Gregor Poynton, and trades union activist Karie Murphy, who is being heavily backed the union Unite.
The union helped pay for a local "consultative survey" on whether the Falkirk seat should be selected via an all-women shortlist (AWS), an outcome that would benefit Murphy, who is an ally of Unite general secretary Len McCluskey.
However, the survey was halted over fears the document was not sent to every local party member.
An internal grouping inside Unite also claimed that the union had signed up around 100 new members to the party, a surge that triggered accusations of manipulation.
It was "concerns over membership recruitment" that prompted Labour to launch its investigation.
Although Unite is not responsible for all the new recruits being considered as part of the probe, it is understood most are linked to the union.
The Sunday Herald can reveal that one of the cases centres around a family in the Falkirk area.
In a letter to the Scottish party in March, an individual explained how he, his wife and child became Labour members: "Myself and two family members have been enrolled by Unite," he wrote.
However, he added: "I or my family did not fill in or sign any forms and wish to know what information the party holds about my family."
He continued: "I have concerns as to the way Unite in Falkirk are recruiting party members."
According to Labour rules, a trades union can pay a new member's subscription fee, but they still have to complete an application form.
A Labour official has already informed up to 90 of the new members in Falkirk that they will only get a vote in the contest if they provided their direct debit details.
Meanwhile, a Unite source has said the union has raised concerns with Labour about an email sent by Gow and Poynton opposing an all-women shortlist.
The source said the email raised concerns about how members' email addresses had been obtained.
However, Poynton said he had accumulated the email addresses over years.
The row is also believed to be part of a wider battle for influence between New Labour and the avowedly Old Labour Unite leadership.
Lord Mandelson said recently that too many selections were "in the hands of one union at worst or a couple at best".
He said contests were being "orchestrated by a cabal of NEC (National Executive Committee) members".
McCluskey shot back: "Mandelson is probably intensely relaxed about cutting democratic corners if it means more New Labour special advisers and the like on the green benches, but utterly opposed to the normal workings of Labour democracy if it means left-wing or trades union candidates being chosen."
A Unite spokesman said: "This is an obvious mistake if it occurred and a mistake that should have been picked up by the Labour Party when it processed the application.
"Unite is confident that its conduct throughout the process has been consistent with the rules of the Labour Party. We hope that the investigation can be conducted speedily and thoroughly."
An SNP spokesman said: "Labour having bogus members in place to be potential candidates in Falkirk – should Eric Joyce stand down – is just the latest sorry episode in this ongoing Labour Party disgrace. Falkirk deserves so much better."
A Scottish Labour spokesperson said: "We have instigated a comprehensive investigation into any membership irregularities in Falkirk and our selection process has been suspending pending this review."
The investigation is expected to be completed by the end of the month.