'The Valley' Cast Blindsided by Dr. Dre, Michelle Saniei Hangout

Dr. Dre and Michelle Saniei have most of "The Valley" cast in the dark ... 'cause TMZ has learned many of her castmates had no idea she was even hanging out with the hip-hop mogul.

Sources close to the cast tell TMZ ... the majority of Michelle's costars were completely unaware of her connection to Dre. We're told the reaction has been pure shock and confusion, with our sources still trying to figure out how the pairing came together.

According to one source, the link stems from mutual friends ... Michelle's best friend is dating Dre's best friend, who works for his record label. We're told Michelle kept things quiet until recently, when it became hard to hide.

While a couple people in the circle may have known, our sources say most of the cast was blindsided and it's worth noting the show isn't currently in production, so all of this is unfolding off camera.

As we reported ... the pair sparked romance rumors after being spotted leaving Nobu Malibu over the weekend ... holding hands.

Michelle's love life has already played out on the Bravo series, from her marriage to Jesse Lally to her more recent relationship with Aaron Nosler. Dre, meanwhile, has his own history of high-profile relationships, including his marriage to Nicole Young, which ended in a headline making divorce.