Toilet level and dark trails

Well, another new fun future project to look forward to - just happened today. Any ideas what could be causing this?
The water level in the bowl has always been constant and has not been leaving any dark trails from water coming from the tank. Now it's leaving dark trails if it sits without flushing for a few hours. And the level is dropping about 1/4-inch every 2-3 hours. There's no leak around the base or anywhere else I can see. Will check it out from the crawl space below tomorrow.
Bowl with no flash
Bowl with flash
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