I came home in the afternoon and my mother-in-law, who is staying with us for a couple weeks in a guest bedroom w/attached bathroom, told me she came back from Trader Joes to discover her toilet full of poop. "I came back and found it this way."

There are four toilets in our house. Hers is the most uphill by at least six vertical inches. From this backed-up toilet down lies her shower, then six vertical inches of drop, followed by three other toilets plus two showers. All three other toilets are perfectly clean. Nothing in the floors of either shower, either.

Her shower, the toilet, plus the garage's W/D drain to the main drain outside then you get a cleanout.

I checked the cleanout: it's not backed up.

So whatever is going on is happening between the cleanout and her toilet.

When I use a plunger on her toilet, it helps the water drain, but then water comes up the shower drain some.

So I am left wondering how in the world her toilet could have backed up while everyone was out of the house to be full of shit, and yet the cleanout is fine, and none of the downstream toilets or showers have had anything back up into them.

My only explanation is she caused a clog but doesn't want to admit it.

Is there a plausible explanation for how her claim could be true given the facts I've shared?

Right now I'm thinking either she clogged it, or somehow a clog just happened in her corner of the house, wrong place wrong time (but again, no one else at home) and it's localized between the toilet and the most-upstream cleanout.

Thoughts? Trying to decide whether

1. to call a plumber for some possible mysterious "shit water getting forced upstream back into the most upstream part of the house but nowhere else" cause I won't be able to fix, or

2. to wait for my sewer jetter attachment for my pressure washer to arrive tomorrow morning, operating under 99% assurance that it's just a clog in the drain near her toilet

   Thanks for any guidance you have to offer.