Edit: oh, it looks like the EFF, Calyx Institute, Freedom of the Press Foundation, NYU Law, the Berkman Center, and a few other orgs run a service that monitors warrant canaries called Canary Watch
Edit 2: oh, looks like they silently shut it down some years ago :( well, we adopted the autocanary standard from FLM linked-above, so it shouldn't be hard for some canary observatory to consume and track our canaries.