We noticed that two of the toilets in our house had somehow deteriorated. One had a low water level with scraps of black material in the bowl. The other had a leak around the base.
So we had a plumber in today. There was nothing technically wrong with the toilets, but the innards needed replacing. One was located on the third floor, which we rarely use. The other in the basement...which we never use.
The plumber explained that toilets need to be used or they deteriorate. Furthermore, this is an old house (c. 1887) and the waste lines were designed for toilets that used massive amounts of water compared to modern toilets. So the measly 2 gallons or so that our efficient 21st century toilets use aren't adequate to keep the lines working properly. We've also seen buildup in the pipes because there's not enough water pushing it through. Big pipes need volume pushing toward the sewer.
The remedy: flush the upper floor and basement toilets even if we aren't using them. If we're in those spaces occasionally, we're to flush them whether they "need" it or not. Just to keep them functioning.