Sometimes your mind comes up with solutions to problems you don't know that you have yet, so the thought may not seem particularly relevant at the time. Later on, that information is sometimes hard to reconstruct intentionally. Sometimes what I do is to go back and repeat whatever it was that I was doing when I had the thought to see if I can re-trigger it. I'd love to have something that I could use to manually re-trigger a train of thought somehow. Something like that could allow you to follow convoluted branches of thought more easily also.
Wow. That sounds insanely annoying. This suggests that, since we don't realize when we've hit upon a cool idea, we should just write down everything we're daydreaming about. Imagine having to read through all of that junk just to potentially find something good you might have thought of.
I suspect that the premise is wrong - we're probably not all constantly having genius ideas that we don't recognize in between daydreaming about dinner and youtube videos. If an idea comes up while I am daydreaming that is useful and relevant, I almost always realize it, even though I may not always write it down. Cultivating the habit of writing these ideas down is useful and necessary, but I don't see how a constantly-pinging app would help with that.