Something I didn't expect were the number of developers that hadn't heard of favicons before. Got quite a few people asking what those icons were. Btw there are security use-cases around them as well nowadays (ex. detecting phishing websites).
No, very often they do add the favicons! That makes it easier to locate websites that are outside of your expected IP space but are pretending to belong to you. For example:
It takes a bit more refining to get a good list of results; the general idea is to find websites that look like the real deal but are located somewhere on the Internet where you didn't expect to find them.