The birthday girl, rocking a Lady Gaga-esque hair bow and pink tutu, was on Team '80s and went from West Hollywood to Santa Monica in search of clues.
But all we wanna know is where was her man?
"I didn't even hear his name mentioned," a source tells E! News. "I don't know why...maybe he was out of town."
Reps for Love and Kennedy didn't respond to our requests for comment, but sounds like Jen had a blast without him.
"They went all over town collecting items, taking pictures and figuring out clues," the source tells us of the bash, which was thrown by Cal Iseminger of Cal's Game Night Productions. "There were prizes, and it was a lot of fun."
The hunt ended at Electric Karma restaurant on Third Street, where she blew out her candles.
Ironically, Jen and Jamie hit the same Indian restaurant together in August and didn't hide their affection during their dinner at that time.
"They were both on the same side of the table, and she was lying on top of him and kissing his neck," an insider tells E! News of their previous visit. "It was quite a scene."
Is it time for her to start bedazzling her girly parts again?
See more stars out in Tinseltown in Party Pics: Hollywood!