You’ve never lived in a small town, I take it? There are no secrets!
I’ve also dealt with organizations that have closed meetings, where people are very interested in the results of those meetings. They can have a meeting and discuss their business— and before everyone has put their chairs away, half a dozen people already know what they talked about, how they decided to act on certain issues, and so on. No secrets there, either!
But actually, that’s kind of a good thing to point out. You run into something similar with the Road to Emmaus story, the Nativity narrative, and things like that. You’ll notice that Luke’s gospel carries very specific elements that aren’t mentioned in Matthew’s gospel, and Mark and John pretty much start with his baptism and ministry.
That sort of insight leads to the tradition that (a) Luke was the unnamed traveler walking with Cleopas on the Road to Emmaus, and that (b) he relied upon Mary’s eyewitness accounts while writing his gospel.
For a situation where they actively have converts (Paul, Nicodemus, Joseph of Arimathea), or even where the Apostles were known to the high priest (like John)— yeah, that kind of information would be very likely to flow, especially since the gospels were written down about AD 66-70 (Mark), AD 85-90 (Matthew and Luke), and AD 90-110 (John). So while we wouldn’t rule out supernatural knowledge, it’s very feasible that the information would have been come by in a very natural way.