He is 5, and fully potty trained. We went to the park (with his brother who is 2) and DS1 disappeared. I went around calling, panicked.
He reappeared and said sorry, he needed a poo. I said ok let's go, but he said he was ok after all. However, he was walking funny and said he had got something on his shorts. I quizzed him whether he had done a poo in his pants, but he said no; I asked what the brown gunk on his shorts was and he said it was mud. (It certainly looked like poo...). I quizzed some more, checked his pants (clean) and we left the park so that he could do a poo. Eventually he stopped lying and said he had gone into the trees, done a poo, and wiped his bum on some leaves. 
OK, so what do I do? I have told him off - there are no loos in the park, and his only excuse was that he said he couldn't hold on. Should I punish? Shoudl I punish him for lying about it? (Thing is, he nearly got away with it - apart from getting it on the hem of his shorts he was clean.) He obviously knows it's not on, or he wouldn't have lied. Er, at a bit of a loss.
p.s. I am totally horrified at the pile of poo somewhere in the park.