Potty Training Regression

Austin, our 4 year old, is completely potty trained. However, since I have put the potty chair in the downstairs bathroom he has decided that it's more convenient than the big potty. I have told him that he is to only use the big potty.

Here's how the conversation went today after I discovered that he had poo pooed in the little potty:

[Me] Austin, what's wrong with this bathroom?
[Austin] Where?
[I point to the potty chair.]
[Austin...hand on chin] Hmmmm....how to solve this mystery! I have an idea!
[Me] There is no mystery here.
[Austin] Mommy, you're my best friend.
[Me] Who went in the little potty?
[Austin] I did. Mommy, you're my best friend! I'm sorry.