we had a 3-yr old that was litter trained when she came to us. She used the box to go pee but never pooped in it.
when we would leave her she would have full access to the litter box. If she was angry at us or if we left her too long she would put her head in her water dish and then proceed to put her face in the litter box. She did this about three or four times. What a mess....
I figured she was trying to tell me it was time to be groomed again.
She would also pee right next to the litter box when she was mad us too. Defiant little booger she was!
It worked well for the middle of the night or early morning when she just couldn't hold it - but I found it to be too messy.