I was reliving an embarrassing moment I had a while back. I used to sell handmade greeting cards at a very chic/whimsical art gallery. The woman that owns the gallery is personable but intimidating at the same time and always made me nervous when speaking with her.
I was on my way to drop off some cards and was running late so stopped at the fast food drive through and picked up a sandwich for lunch. It was some chicken concoction with mayo and shredded lettuce. I walked into the gallery and talked with the owner for a spell after handing her my cards. Me, nervous as ever and she was looking at me a bit funny. I get out to the car, look in the mirror and there's a blob of lettuce and mayo on the side of my face. It had taken on a mucous-like quality from the heat of my blushing cheek.
I was embarrassed beyond belief which turned to frustration at her lack of common courtesy. Why didn't she say something? All it would've taken is handing me a Kleenex and a hushed "Here, there's a little something on your right cheek."
If someone I know has some mustard or other foreign object on their face, I discreetly let them know. I think it's common courtesy. For myself, I usually look at myself in the rear-view mirror before I get out of the car and perform routine foreign particle checks.