No one has used the sinks in the last day, that is for sure. The laundry is downstairs, not up; and the water that comes into the sinks is not soapy. It is slightly yellowish water that leaves a dirty, grity residue when it drains out (right now it is dry again!). Another bit of information that I forgot to mention earlier; the faucet on the left sink has been removed (needed replacing), so no water could be run in that sink. Yet both sinks back up with water, even though they do not share a common waste trap!