Bell icon in corner. Colored dot on bell. No need to overthink the UX on this beyond what color to make the dot.
This is especially irritating when, say, you set up a new phone and the app treats you as if you've never used it before.
Really my complaint is anything that covers up content; if instead of popping up a popover Firefox just took 75px above or below the page to show me something I’d complain about lot less — but if I had my way anything unwanted that covers unwanted content should bust down the whole c-suite to working in an Amazon warehouse. (I could trust those folks to deliver stuff with an e-bike but don’t want anybody with bad judgement like that driving a car or truck!)