yep can't say that i have seen it.
we get this question on a daily basis.....so here is my spiel:
there are millions upon millions of apps each with their own unique icon for notifications. add in that websites can also send you notifications and they too have their own icons as well. so you see showing us a random icon and ask us what it is, is almost like predicting what the next set of lottery numbers will be. it is like finding a needle in the hay stack.
the only way to know for sure is if you look at the phone's notifications. but 9 time out 10, the phone is not accessible for one reason or another.
so with that said, you will only get a guess from us and not a true confirmation of what the m logo represents.
.....good luck