No Problem. As someone with a severe sensitivity to light I wear sunglasses virtually everywhere. I have a tinted (not dark enough for my liking) pair of prismatic glasses I wear in front of the computer, a different tinted pair for watching TV, another pair for dealing with street lights etc at night and my dark polarised sunglasses for everywhere else. On a sunny day I could probably walk down the street in welding goggles! I never open the curtains while I'm at home, only opening them to air the house when I go out. Despite them claiming to understand, I know this drives my housemates nuts but what choice do I have - I've already had headaches/migraines every day of my life, I can't afford to make them worse. The only time I take my glasses off is when I'm reading, provided there's no reflection off the page, as it strains my eyes.
I would be suspicious right off the bat. I would have to size them up. Are they punk teenagers slouched in the chair? I would come to the conclusion they are making people uncomfortable to get a reaction. If they were older or a mom with a child - I would be a peace with the fact that they probably have a medical reason for doing so. If it's not for medical reasons then it's rude to where sunglasses inside. There have been times that I had to where them inside for health reasons and I would tell everyone around me I had my eyes dilated or whatever to set those around me at ease.
One feel vulnerable in a room with someone wearing sunglasses, it would be like someone wearing a ski mask. By the way Banks and other public places will not allow people to wear hats, ski masks or sunglasses upon entering for that every reason.
Well it also depends where inside, and what kind of sunglasses/what kind of person. Like, if you work at an outdoor pool but you wear your sunglasses while you're in the break room doesn't mean you're vain, I work at a pool and even the guys wear their sunglasses inside. I don't think it means they're vain otherwise, but again it all depends on the situation.