I've noticed this in the past and have noticed this recently too. A globe/gate valve has scale buildup (near a leaking packing nut). I tried tightening the packing nut but it wouldn't budge. The valve still leaked small slow drops, even after a few days. But I noticed after a few weeks, this leak was non-existent. Have you ever noticed this? Is this because the minerals/scale in the water have essentially plugged the leak?