feces outside of litterbox

I have 2 house cats, brother and sister,about 5 yrs old. About once a month or so I find a small amout (like a single "pellet") of feces in the house- never in the same place - on the furniture, bare floor, or rug. Often in the center of walking area. The cats are overweight, but otherwise healthy. Other than that, they share a litterbox just fine.
Has anybody had an experience like this or know what can be done about it?

We call these dingle-berries!
Sometimes a dingle-berry will get stuck in the cat's fur (or it just doesn't quite make it all the way out when they poop) and then it later drops to the ground as they are walking around. I tend to see more dingle-berries when the cats are slightly constipated and remedy this with a little pure canned pumpkin or cat lax.