Former Scottish minister George Lyon, who in public is fighting for his re-election on May 22, attended a meeting for quitting MEPs last month, the Sunday Herald can reveal.
The event, at the European Parliament's Jozsef Antall building, was intended to offer financial advice to MEPs who are standing down this year.
Lyon asked a question about "transitional allowance", the golden goodbye given to departing MEPs to help cushion their exit.
The LibDems said Lyon's attendance was part of "normal contingency planning", and that he had attended on behalf of the LibDem group.
However, the Sunday Herald understands other LibDem MEPs also attended the meeting.
A Brussels source said Lyon had looked "sheepish" at the lunchtime event.
An SNP spokesman said: "The appalling damage that the Westminster Government has inflicted on communities across Scotland has only been possible because of the LibDems' shameful decision to prop up the Tories.
"While George Lyon clearly expects to lose his seat at the coming election, his concern over tax arrangements suggests he doesn't want to pay the price for his party's actions."
With the polls showing a collapse in LibDem support since the party entered Coalition with the Conservatives in 2010, May 22 is expected to result in a cull of Nick Clegg's MEPs.
Lyon, 57, is already the most vulnerable of Scotland's six MEPs, as he was elected in sixth place in 2009 with only 11.5% of the vote.
If Lyon fails to get re-elected, the SNP are most likely to pick up his seat. Ukip is tipped to win the most MEPs in the UK, followed by Labour, with the Tories third.
LibDem leader Nick Clegg praised him at March's Scottish LibDem conference, saying: "We're all behind you, let's get you re-elected."
A LibDem spokesman said: "This was an entirely routine meeting on staffing arrangements and issues relating to the end of the Parliamentary term attended by hundreds of MEPs from all parties. George Lyon has delivered for Scotland in Brussels. We are confident that he will keep the seat and continue to stand up for Scotland at the EU."