Let me introduce Invader. Invader is a street artist who has been creating mosaiques (invaders) in public spaces for decades. For instance, there are about 1 500 invaders in Paris.
Photo by Robert Katzki on Unsplash
Something great about Invader is that they published an app allowing to take pictures of invaders and claim “points”. I enjoy it a lot, as an incentive to walk often and in various places.
Now the app may not be very comprehensive, but there are also community efforts. For instance, here is a map I found.
The map is not perfect either, but one can build upon it. Here is an example.