I think it's pretty common. DS didn't hide it quite to that extent, but we were still surprised when we found out just how much he could read at that age. His preschool teachers didn't know he could read at all until we told them.
If you want to know how much they can read, a sneaky way might be to let them read to a pet and then eavesdrop on them. DS used to like to disappear into the basement with our dog and read to her. Our local library has read-to-a-dog programs for reluctant readers, but DS did that on his own even though he was not remotely a reluctant reader.
