Nelson Piquet Jr. has just let it slip about Verstappen and Mercedes being in talks...

https://www.reddit.com/r/formula1/comments/1lwegqn/piquet_jr_on_brazilian_podcast_discussing/

He presumed everyone knew about it already and says the final contract details aren't decided yet. And he adds that Verstappen's teammate will be Antonelli in case of an agreement.

Things are getting more concrete slowly then... This is absolutely the start of a downfall for Red Bull.