My DD is 3.5, she was potty trained back in August/September. She doesn't have any wee accidents at home anymore but has now taken to pooing in her undies when she's playing and doesn't want to stop. She's still wearing a night nappy and will poo in that if it's not taken off as soon as she gets up.
She used to poo in the potty and still will sometimes so I know she's not afraid and she's not constipated. She doesn't tell us when she's had an accident and doesn't get upset. She's not bothered by the poo and would sit in it all day if we left her. She runs off when I try to clean her up and throws a tantrum if I try to put her in the shower.
I've asked her why she does it and she just says 'I don't know' she knows poo should go in the potty.
I really think she's just being lazy and I'm not sure of the best way to handle it, mostly I just stay calm and clean her up. Although I'm about to try bribing her. I considered time out and making her clean out her undies herself.