A seemingly knowledgeable guy who davens with me early each morning is sometimes found in the women's section (which is usually empty during that service). He says that he learned that from a "little-known" halacha at Shulchan Aruch § 109, which he says requires that he daven separately from the congregation (either in the hallway or in the women's section) if the congregation has begun the Shemoneh Esrei before him. Otherwise, he says, he has to wait for them.
I looked at the S.A. § 109 as included in the Mishna Brurah. My Hebrew isn't that great, so maybe I missed something, but I couldn't find anything there that told me he had to separate himself from the congregation. From what I read, it is more of a discussion about catching up methods and priorities. Did I miss it, or did he actually get that from a gloss on the S.A.?