"tunnel that connects a larger module to a docking port" - reads like stress-induced metal fatigue, where it leaks a bit from every seam. It may not be a nice round hole somewhere.
Or just not an ideal design and the welded ring at the end is where it comes to a head because that's where the part connects to the next one.
All aluminum stuff is gonna suffer fatigue problems eventually.
And being a docking module, I'd guess it'd be near the docking port structure (although all Russian modules are connected via docking ports, rather than berthing ones).