I saw it for the first time last night. I think it was Vimeo. It was actually much better than the "try our app" popup, because it was android asking if i wanted to use the instant app, and when i said no it just went away.
That's more reasonable. I wonder if these work via Firefox? Maybe that's why I've never seen them.
I normally use firefox too, maybe I accidentally opened the link in chrome last night. I don't have my phone around to test. I just remember thinking "ahh that's kind of neat" before clicking no.
As long as it remembers no locally and just lets me use the web app forever after it's a godsend.
They should't be available via Firefox, just chrome/google search