2 Year Old Housebroken Cockapoo Has Started To Urinate In The House… What Happened?

Our 2 year old male cockapoo has been wee-wee pad trained since the beginning and we also take him outside to go to the bathroom. However, the past couple of weeks, we have discovered him making little accidents on the carpets around the house. He is still relieving himself on the wee-wee pads primarily, but randomly finding spots where he has peed every few days or so is disconcerting. Why is this happening and how do we fix it??