"I thought he was potty trained"

Yesterday in the nursery at church, Matthew asked to use the potty. The worker, assuming he was trained since he asked, took him to the potty and he did everything right. She thought it was strange that he wasn't wearing underwear. When we arrived after church and were getting him ready to leave, he dropped his pants for Daddy and announced that he wasn't wearing a diaper. Daddy frantically searches his pants for a diaper that may have become loose. Nothing.

Daddy: "Where's your diaper, Matthew."
Matthew: "I don't know."
Worker: "He asked to use the potty, so I thought he was potty trained. Thought it was strange that he didn't have any underwear."
Daddy: "He's not potty trained. He was wearing a diaper."

We do a quick scan of the nursery space. No diaper. We replace the missing diaper- not safe riding in the car without one. Today, Mommy and Matthew are off to Target to get disposable training pants for school and a package of underwear. He's very excited.