I only transitioned two months ago, and I had a 3 hour conference with someone last week. Apparently she had no idea I was trans, looks, voice, mannerisms. Until she saw my shoes and realised my past because I had big feet and it all then slotted together. Feel a touch insulted as my feet aren't even big for a girl of 5 foot 7 (UK size 7-8) but I'm told that was the straw that broke the camels back as it were.
Seems to me that passing is not a "one thing" thing but rather holistic. There may be a few tiny little things that add up to a giveaway. I'm sure she wouldn't normally look at someone with size 7-8 shoes and think of them as trans, but add in the other little clues, a face that has a few masculine traits being pre-FFS, a voice perhaps a touch deeper than normal, or without the correct inflection, and suddenly the alarm rings.
Obviously there are a few "one thing"s that are an automatic giveaway of gender, a beard for example, but for most other things, height, broadness, voice, feet, shape, mannerisms etc, its more part of an overall parcel. Deal with as many as you have control over and don't worry about the ones you can do nothing about.
My thoughts on it anyway
Em