When you make a plumbing joint with screwed compression fittings, there’s always a question of whether you’ve tightened the nut enough to make a good seal. The fittings come with copious warnings to not overtighten the nuts, which means I tend to undertighten them.
It’s easy to spot a major leak or a trickle, but what about a very slow ooze on a hot water line where the drip will evaporate before you notice?
Fold up a piece of tissue paper and secure it around the joint with a wire tie. Come back a few minutes / hours / days later: it’s easy to tell if the tissue has ever been wet, because its texture will be dramatically different.
Having just replaced a water heater, the subject of leakage is a hot topic around here…