That's what I figured. As far as I know, no anti-leak was used. This is probably the original cap (19 years old) and the O-ring's probably just worn out. Thanks for the advice.
Before I go long-term cruising, I will remove and pressure-test the heat exchanger and will change all engine hoses, and keep the originals as spares. Right now, I'm just looking to avoid refilling the coolant expansion tank every week.
U.S. and Canadian dollars are near enough par at the moment for it not to be a consideration.