DS got the BFE out of the blue, with no warning. I don't think he looked at his portal for some time after, so no telling when it actually showed up there.
Last year I found out first when I opened up the BFE - it was a nice surprise and I'm glad I got to experience it that way. This year I found out first through my portal being updated (that happened about a week ago and I haven't gotten the BFE yet).