This is quite common, except I suspect it's more a case
of it only showing when system is cold -- when system is
hot, it dries up as fast as it leaks.

Not quite, because the water pressure never dropped while the system
was hot - it could run for days (and nights obviously) with no trouble.
Only when I let the system go cold did the leak appear.

As it happens, some parts of the install are using PE-X plastic pipe,
and I think they are moving when heating/cooling and had highlighted a
poor joint when they cooled, and moved again.