Interpretation. This answer is an interpretation or an open question.
Scripture does not say how the disciples learned about these prayers. It is a funny thought, someone listening in on a talk between the Most High and His Son. But that is most likely not what happened. It is more likely that the Messiah told the disciples Himself. He walked with them for at least a year, so He could easily have explained it, just as He explained so many other things. And even after His death, He appeared to them for 40 days and kept talking with them about the things of the Kingdom (Acts 1:3).
Another possibility is that the Spirit showed these things to the disciples so they could write them down as a testimony for the people (John 14:25-26). The Spirit of God was promised to teach the apostles all things. It makes good sense that this included the Messiah’s private prayers. There is also a pattern for this. The Spirit is how the prophets long ago had the Word revealed to them (2 Peter 1:21).
So no one needed to listen in.
Answered by Taught of Yah
