Is gray stain in vinyl flooring around toilet *always* a wax ring leak? [View all]

I've got a gray stain developing at the back of one of my toilets in the vinyl flooring. It's a wood subfloor with crawlspace. Inspection beneath around the pipe in the crawlspace shows subfloor dry and stain-free (so far). No sign of moisture damage there.

All I read says yank up the toilet (which is solid and not loose or wobbling) and the flooring and replace the wax ring, flooring, and whatever wood might be damaged beneath. I am not much of a DIY so this means $$$$$ that I don't have.

Is there anything else that might cause this? It is a small bath with shower next to the toilet and a regular shower curtain (not a glass door). The caulk at the floor where it joins the tub is good - not wet, peeling or coming off and no sign of water at that location - just gray at the back of the toilet which isn't even a foot away.

One plumber has said he thinks it's from the shower but even if it is, WTH can you do? Can't get rid of it since it's under the damn vinyl flooring, not on it.

*sigh*

EDIT: PS - any way to more positively diagnose the damn issue here without tearing up the effing floor?