On the internet, you can search, crawl, observe and monitor public human expression and activity. There is no universal index or central registry, but you can see and find a lot with automation.
In the real world, the landscape is blurrier and more opaque. You can't singularly search, crawl, observe and monitor public human activity globally at a micro level. But the internet still exists in the real world via QR codes. And while there are no interactive links in the real world like on the web, QR codes are the living instantiation that provide the direct link from the real world into the web. They make this connection with all of their valuable physical-world context — geolocation, microlocation (restaurant, lamppost, toilet), surroundings, date and time, purpose, intentions, audience, legitimacy, lawfulness, authorship, provenance, security, privacy, visibility and overtness.
QR codes are the graffiti of the internet. They are living objects, interactive gateways and point-in-time artifacts. And they deserve to be studied.