After seeing this leak appear a few days in a row in the toilet off of my bedroom I believe that it is time to replace the toilet tank gasket. Thankfully this is an easy task, requiring nothing more than disconnecting the water supply, drying out the tank, unbolting it from the bowl, reversing all the steps after fitting a new gasket, and having a test flush.
I must remember to pick up two new gaskets tomorrow, as I shall replace both toilets’ gaskets at the same time. They were both installed at the same time (October 2001) and should wear based on age and exposure, but not use. Utilizing, cleaning, nor flushing the toilet should cause little wear on a gasket which normally sits just below the flapper and is dry save for during flushes. Thus as toilet usage patterns can be ignored when considering gasket lifespan, one can presume the other gasket is likely to fail soon as well due to simple effects of aging.
(Note that in presuming even wear between the two toilets I’m ignoring differences in the bathrooms such as airflow patterns, temperature, exposure to light, humidity, and the possibility of someone leaning back while on the toilet introducing stress to the gasket, as all of these have uncertain outcomes and would be much harder to quantify than simple age. And anyway, who really leans back against the bowl while sitting on the toilet? Things won’t come out right if one does that.)