I've seen this or similar questions. Occasionally, over the past month or so I'll get a notification on my phone. It's a chime like a bicycle bell. A little "ring-ring." It's only comes through maybe once a week. But I get no visual indication of what it is. I've searched through all my apps that have permission to notify me, and nothing is set for that sound. Any ideas? It's not a biggie since it's only once every week or so, but still... Lol