The actor made an appearance at Manhattan piano bar Marie's Crisis for a singalong last Thursday (01Oct15).
He was joined by his former TV co-stars Samantha Ware and Alex Newell, according to New York Post gossip column Page Six.
An onlooker tells the publication, "They were singing with everyone in the bar. I was basically in Glee! It was so cool!"
They reportedly belted out tracks by Billy Joel as well as several musical numbers.
Darren, who appeared in hit stage show Hedwig and the Angry Inch, recently launched a Big Apple music festival featuring Broadway's biggest stars.
Elsie Fest, which took place last month (Sep15), was a one-day outdoor event celebrating songs from the Great White Way.
Darren took inspiration from the popular Coachella music festival in California.
"I was at Coachella and thought 'Let's do this for show tunes,'" he previously said of the venture. "Musical theatre has been going through a wonderful shift in the past few years, from the type of work that's being produced to the way it's being consumed by the public.
"It's more than showtunes on Broadway - it's songs from film, TV and YouTube. Since the rules were already changing, I figured, why not shake up people's ideas of where to watch the performers they love. Enter Elsie."