I can assure you that you have encountered this type of person. I'm one. At least half the active members on this forum are introverts, married or otherwise. Between one-third and half of the general population is introverted, which means unless you have a very unique workplace a solid chunk of your coworkers are.

The problem with introverts is that we by definition don't stand out. When you notice us, it's because we're being uncharacteristically social, so you assume we're just like you. When we're in our comfort zone, you won't see us at all and so you once again won't have any evidence that there are actually people out there who prefer to be alone.

One key to discovering introverts is to not assume that "introverted" means "[not] smooth with people in all sorts of contexts". At work I participate more than most of my teammates, and I'm really good at understanding people and helping build consensus, but it takes a lot out of me. If you just knew me from work you might not realize it, but by the end of the day I don't want to spend any time with anyone other than my wife.