Welcome to Android Central! You can find out what app is responsible by swiping down the notification panel and looking at the details of the notification. If this is not your phone, then ask the owner of the phone.
Notification icons tell you only that you have a notification. Users are not expected to know what they mean, and certainly not to know what the notification itself is from a mere icon. How would that be possible? That is why it is wise - though purely optional - to swipe down for details. At any rate, that is normal practice in the use of smartphones.