Ive seen numerous people tell a cop "do you need me? no? Okay." then get out of the car and walk away.
It's my personal opinion that it's a shade a grey in reality.
For instance:
If the probable cause for the stop is nothing more than a traffic violation, the passenger shouldn't required to notify.
On the other hand if there is weed smoke pouring out when the officer walks up, the probable cause circumstance just got changed and the officer is performing in an official capacity with all occupants until he/she gets things sorted out (he/she may or may not arrest a passenger). Until then all occupants would be required to notify under the law. Edit: Actually I think this is the case regardless, but I don't know for sure.
Do what you will, but I myself don't care to be the matter of a test case.