Let me tell you that this happened to me more than once.
The calls were just like yours and I said do you have little boys ?
I knew what the cause was, but the customer new better.
The answer was " my little boy doesn't do that anymore " so while I was there I reset the toilet with a new wax ring.
I made two more calls there again. After I left the last time she called the shop to say that she found her son using the toilet and saw him whizzing and not all of it went into the toilet.
I found that some young mom's attitude was " my little boy doesn't do that "
But how many times that happened and the mother didn't have the courtesy to call and say that she discovered what really happened, who knows....
Don't know why they would check the septic at all. You would have other things going on.
Nix on the vent too...........
BUT it coulden't hurt to re-set the toilet, thats not hard to do and while you have the toilet loose, get some hot water and a lot of suds and go to town......