A two-year-old boy wearing only his pyjamas and a nappy managed to travel 28 miles on a bus from Wales to England without anyone noticing.
Two people were later arrested on suspicion of child neglect and have been released on bail.
Inspector Jonathan Tatton of Dyfed-Powys Police said:
“Fortunately the child didn’t come to any harm, and a thorough investigation is now under way to establish how this occurred.”
He added: “Following the report two persons have been arrested and remain on police bail pending further enquires.”