I've noticed a few times recently new notifications popping up which I was pretty sure I'd seen already. I just got another one and this time I've captured some data. I got this alert just now on commons (the bell icon turned red):
Schwede66 mentioned you on the talk page of user Krinkle in "KrinkleBot is skipping the English page", with a timestamp of "1mo":
That event did indeed happen 1 month ago. The page in question hasn't been edited since July 28th (1 day after the original alert).