Boiler developed slow leak last spring. Last spring I closed both valves for in and out pipes. Boiler is a buderus logano g124
Today disconnected top pipe from boiler no fluid escaped. Disconnected bottom pipe several gallons came out. System has not been active since spring. Is it a logical place to expect the leak from the top pipe fitting or could it be the bottom. Back of boiler Is closed in tin case and difficult to determine by residue as the top pike had a leak about 4 years ago and was repaired then but evidence of that leak is still present also both pipes are in line.

Stands to reason that if you disconnect the top pipe you have broken the vacuum (taken your finger off the top of the straw as it were) when you disconnect the bottom pipe the miracle of gravity does the rest.

I hope that helps you understand why no fluid escaped the top connection but as to where the leak might be nothing in your question gives any real clue.

Journeyman plumber/pipefitter, employed in piping trades since 1989