It is a pretty humorous thought to eavesdrop on a conversation between the Most High and His Son, but I doubt that was the case at all.  It’s more likely that the Messiah told the disciples Himself.  Remember He walked with the disciples for at least a year which he could have easily explained it to them like He did so many other things.  Don’t forget, even after His death He appeared to them for 40 days and continued to discuss with them the things of the Kingdom [Acts 1:3].

Another possibility is that the Spirit revealed these things to the disciples to have them written as a testimony to the people [John 14:25-26].  The Spirit of God was promised to teach the apostles all things and it is beyond reasonable that the private prayers of the Messiah were included.  There’s also a precedent for the Spirit revealing things to people because that is how the prophets of old had the Word revealed to them [1 Peter 1:21].

No need to eavesdrop.