“I’m not in the room” is the present tense of “I wasn’t in the room” and is an extremely common American idiom. The very popular American broadway musical Hamilton even has a whole song devoted to wanting to be in the room. The room is commonly understood as the place where important topics are discussed and important decisions are made, usually alluding to a greater body of authority. Example: I think the government is considering raising interest rates but I don’t know, I’m not in the room.

It probably means the speaker literally isn't in the room while the discussion was taking place. It would be more likely said in the past tense: "I wasn't in the room." Literally, it means I wasn't there when they spoke on this topic and/or when they made a decision about it. More metaphorically, it means something like, "I am not included in that discussion." Again, here I'm using the present tense because your sentence is in the present tense, but typically you would see this in the past tense.

The speaker is not only saying he doesn't know, he's suggesting that other people who are talking about it don't know, either. "It's anybody's guess" means that everybody is just guessing, and nobody's guess is any better than anybody else's. "I'm not in the room" means literally that the speaker is not in the meeting room where the decisions are being made. But it also carries the idea that only the few people who are actually making the decisions know, and that anybody who isn't "in the room" is just guessing.

Hi! I'm a native English speaker from the UK. "It's anybody's guess. I'm not in the room" is like saying "nobody really knows. I definitely don't know" but it is very uncommon to say a phrase like "I'm not in the room" we would just say "I don't know" hope this helps :)

would + will is incorrect English I’m not in the room is literal translation from another language. I guess it means just - I am not on hand, how could I know?