Alright - long story short, kitchen sink was backing up and slow draining. We replaced the piping and my husband & father were making sure everything flowed smoothly by running water down the kitchen drain. There is a clean out pipe attached in the basement that leads to the city sewage line for dumping.
Dad dropped the cap to Draino down the clean out. They fished for a while, trying to flush it out, no dice. Figure that it lodged itself sideways in the pipe because no water was backing up. Well, today it started backing up again, taking a lot longer to fill this time, so my assumption is the cap dislodged partially and is now blocking most of the pipe (water does drain, just takes a while).