Legal Question in Criminal Law in California

An arrest is unlikely for simple possession that the police did not see. But you could be charged without being arrested. Nothing you've written suggests that this is likely. Presumably, though, you're asking because the situation might come up again.

There's no firm rule about who can be charged in situations like yours because each case depends on its own facts. What matters is whether there's probable cause to believe you possessed some of the pot. That will depend on all of the circumstances, not just the ones you've described.

Just being in the room when other people are smoking pot doesn't amount to possession. Possession involves exercising dominion or control over a drug with actual knowledge of what it is. Proximity isn't enough to make you guilty.

By the same token (no pun intended), distance isn't enough to make you innocent. It is very common for more than one person to simultaneously possess the same item, even when one has more access to it than the other. Note too that possession is not the same thing as ownership. People often possess things that other people own.

That the pot is now in your roommate's bedroom is relevant, but it doesn't prove either that he's guilty or that you're innocent. After all, the pot that was smoked on Saturday isn't anywhere at the moment, but somebody possessed it at the time. And drugs that are in your roommate's room right now might have been in your hands on Saturday. If the police have good reason to believe that you possessed the drugs, the interests of justice would not be served by a rule that lets you off the hook because they're somewhere else now.