it also does not have to be just an app notification. websites, if you grant them permission to send you notifications, can also use icons similar to what you have.....so that adds many more icons that they could represent.
we get this question daily, it is really hard to help majority of the time, because there just way too many things that they could be.
the easiest way is to just look on the phone, but then that would be sneaky and going behind someone's back which is not good for the relationship. or you can just be open and ask....but that is not for me tell you how to do that. i am definitely not a relationship expert.
.....good luck