I suppose it started when Jane commented that our water bill was higher than usual one month. Then there were these odd moments, during the night while laying in bed, and I could hear an ever-so-slight trickle of water. It was difficult to decipher because we sleep with a sound machine, and I believe at the time we listened to it rain all night long (most of the time it is ocean waves). Finally I decided to take the top off of the toilet in Jane's bathroom and... sure enough... there was a leak.
I have repaired toilets many times before. Shoot, I've even replaced them. However, for some reason, my first instinct seems to always try that, "Let's just ignore it and see if it goes away" approach. That rarely works. So, I started by just turning the water off at the toilet. The water drained out of the tank in less than an hour, so that indicated it was likely the flapper that was the issue. I pulled it off, and it wasn't cracked or broken, but it had grown a bit deformed in one area. I went to Menard's and looked over their menagerie of flappers and finally settled on this $5 one instead of a $4 one (or a more expensive one). I don't really know why - probably because it looked better than the others, and had that little yellow sponge-thing. And... it worked. So there is no more leaky toilet.
It feels good to have actually done something right for a change. However, I am a bit ashamed that I didn't do it sooner. Leaky toilets can really drive the water bill up in a hurry. So... lesson learned.