Coolant can travel up hill and down as you pointed out by capillary force.
Volkswagen had a notorious design gaff whereby coolant could wick from the coolant expansion tank throughout the entire wiring harness due to a poorly positioned probe. More than a few unfortunates discovered coolant dripping out of their tail lights.

Heat cores are thinly constructed for maximum heat transfer. As such anything on them can go. If you are going to the trouble of taking it apart take it all apart and test the core even if you are sure it's just a hose.

'89 P'up, 2.6 I-Tec, 488,000 miles and done... gone to the great beyond