Loremaster:
You preferring it doesn't make it "correct."
I've seen it cause problems many times, especially when games are first starting out. There are other ways to do it, but the GM ultimate establishes the correct format for a given game. Even if the convention is only established by weight of everyone else, if you have one person posting in limited and four people posting in objective, the person posting in limited is not conforming to the group and thus "wrong".
If your group all decides to go with third person limited, then knock yourself out. I wouldn't object to you doing so, but I'd probably also prefer not to play in that game unless other factors outweighed that.
Other points:
Being alone in a room is something I rarely see played out in regular threads. It's usually done in PMs, PLs, or otherwise hidden threads. I'm in a game right now where I'm doing something similar... but it's a private thread (and I'm posting in third person limited because of it). I wouldn't call it "wrong" to do otherwise, but I prefer compartmentalization of knowledge in games for a variety of reasons.
As for communications to the GM... that's what OOC lines and private lines are for. If Sarah is looking for something, she needs to call out the game effect she's going for in an OOC or PL. GMs read and respond to a whole lot of text and, depending on the GM and the game, can easily miss things. Clarity is critical, and relying on nonverbal cues that might be buried in the middle of a wall of text is just a bad idea.