Pokémon GO Special Incense is a feature that is being tested for a some players. You might have gotten a surprise visit from Professor Willow and obtained a Special Incense. What does it do?
Pokémon GO Special Incense: Get Free Pokémon and Items
The Special Incense won't appear in your bag. Rather, it will always be on once you got it. It will give you a Pokémon encounter that only you can see, once per day.
To get a Special Incense, you'll just have to be lucky and be selected. Niantic is rolling this feature out slowly, and they're testing it with a small group of players first. Niantic says that this is part of "continued efforts to adapt Pokémon GO’s gameplay to individual settings."
In addition to the Special Incense, Niantic is experimenting with daily free items. Some players might get a bonus item box from the shop. Getting the Daily Free Box is like getting in on Special Incense: you'll have to be one of the lucky ones chosen to test the feature out on.
The box contents aren't anything too flashy, so you're not missing out on much if you weren't selected. Players have reported getting a couple of Poké and Great Balls, along with some Potion.