The 25-year-old pop star arrived at The Roxy in a blonde wig and sunglasses at about 11 p.m. Sunday – but once inside no one seemed to notice the still-shaded Spears, who went into the VIP bar On the Rox.
Spears and her friends "sat by themselves," a source tells PEOPLE. "No one noticed her – so much so that no one ever went over to her and offered her a drink or anything. ... She was wearing sunglasses for at least part of the time she was inside, which might have been why."
A private birthday party with about 80 people was taking place in the VIP section, which also offers karaoke. Spears stayed for about an hour and a half. Her cue to leave? When a partygoer having a turn at karaoke chose Spears's song "... Baby One More Time."
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Adds the source: "I guess it was a bad rendition of the song or something, and she left."
The New York Daily News also quotes a source saying Britney was spotted sitting alone in a black leather booth at the club.
Meanwhile, over the weekend the owners of Esther's Haircutting Studio – the Tarzana, Calif., salon where Spears buzzed off her hair – put several Spears items up for auction on eBay. The Web site later yanked them.
Undeterred, the salon owners set up BuyBritneysHair.com, asking for offers of $1 million or more. The package for sale includes Spears's hair extensions, the Omega clipper used to cut it all off, the Red Bull she drank at the salon and her Bic cigarette lighter. The owners say that some of the profit will be donated to charity.