I was playing skyrim on a new file, mainly just wandering around, and I suddenly heard an argonian asking if I wanted to buy something. I looked around but found nothing. When I used console commands to check if he was underneath the ground, I got the same results. However, when I was going back up towards the surface, I saw the phrase on my screen that indicates that there is someone to talk to even though I couldn't see anyone. I eventually found the invisible npc and talked to him only to have him ask me if I wanted to buy skooma!

This is my first interaction with a skooma dealer, so I'm not sure if this happens often.