OK, you're right, doesn't seem to have anything to do with synctex. I realised that even without compiling, every time I opened a .tex file I started to get memory leaks (quicker when I scrolled up and down through the text). It didn't really matter if it was a luatex file or not -- luatex is a red herring (it was simply that I've been mainly using tex via org, and so hadn't noticed the leak before)
After turning on and off various things in my .emacs, I finally "fixed" the leak, by disabling "preview"
(Specifically by commenting out: ; (require 'preview-latex) ; (load "preview-latex.el" nil t t)
I don't use preview anyway, so this is a fine solution for me. I'm not quite sure why preview is causing the leak, though I suspect perhaps there's something idiosyncratic (and wrong) about my .emacs setup.
Also, this also plugged the leak under GNU Emacs 23. I tried the snapshot of Emacs 24, and I still have a leak there (though I don't think it's AUCTeX-related).