This is probably a good idea. I hesitate with anonymity because it’s possible the owner wants to explain their side of the story, or maybe why they had to leave their dog outside. Sometimes there’s a good reason. Someone once posted an anonymous letter on my door complaining about a noise that wasn’t coming from my place and I wish I could’ve explained to the person that it wasn’t me. I guess there’s too much risk confronting and politely asking a neighbor to help you out though these days