The messy truth is that many children with encopresis hide their soiled underwear. Generally, they do it to postpone getting in trouble, even if their parents have told them many times that they will not be upset with them for having an accident. They often believe their parents will also be upset or disappointed with them for having messy underwear -- consider the look on any parent's face (not to mention the "eeew/gross" commentary that often accompanies it) when presented with such underwear!
Typical hiding places include under and behind their bed, in their closet and behind the toilet. Curiously, some children try to hide their soiled underwear at the bottom of the dirty clothes hamper.
One technique I recommend to prevent this behavior is by putting all of your child’s clean underwear “under lock and key” and then telling him that he needs to turn in the underwear he is wearing in order to get another pair of underwear. This will need to happen on a regular basis (i.e. before PJ's, before baths, in the morning, etc).
Another option is to allow the child his own access, but only provide seven pairs and number the underwear 1-7 (or label by day of week) to ensure you've gotten all of them back at the end of the week.
When your child does bring you his underwear, it is important that you praise him for doing exactly what you asked him to do even if his underwear is soiled. Some children may need a small reward or to have this action featured on a reward chart. You want him to learn that you mean what you say when you tell him that it’s OK if he has an accident.