Yellen appeared before the House Financial Services Committee to give remarks on the state of the US economy and field questions from committee members. As chief of the US central bank, Yellen also spoke about a recent semi-annual report delivered to Congress by the Fed.

And while Yellen made headlines by expressing her willingness to raise interest rates amidst a healthier economic climate (following years of near-zero rates instituted in the wake of the 2008 financial crisis), it was one attendee who drew notice on social media after he held up a "buy bitcoin" sign two rows behind where Yellen was seated.

A screen grab of the moment was tweeted by CNBC staffer Steve Kopack, and later shared by others who have been watching Yellen's testimony before Congress.

The identity of the person who held up the sign isn't immediately clear.

Yet in a follow-up tweet, Wall Street Journal national economics correspondent Nick Timiraos reported that the individual, along with another, had left the room "after a staffer made some instruction to them." Video published by Bloomberg shows the two individuals being spoken to by a staffer.

Yellen – who told Congress in 2014 that the Fed "doesn't have authority to supervise or regulate bitcoin in any way" – remarked earlier this year that she believes blockchain is an "important technology".