How to occasionally push notifications to mobile applications?

Question

I have a use case where, when when suspicious activity happens on a user's account, I want to show them a pop up in their mobile application. This pop up should appear as soon as they open their app, or if the app is already open then this pop up should appear with in a few seconds of the event happening. This event might happen say once in a few months. The user has to click on one of two buttons in this pop up. Long polling and event sourcing both seem like an overkill. Is there a better less resource intensive way to achieve this?