Dirty water in toilet bowl after sitting

I had the same thing happen to me just a few weeks ago. The water in the bowl was clear when we parked it, but brown when I checked on it a few weeks later. No smell, though, thank goodness. Never saw that happen before. I recalled that we hadn't cleaned the toilet after the last trip, so scrubbed it out with a chlorine cleaner and went on my way. Problem hasn't occurred since, so I'm thinking that there was some leftover bacteria that bloomed over the few weeks after we got back. Might be what's happening with yours, too.