When you leave a placed stained glass and come back, sometimes, it will start to give off light. The light level stays 15 inside the stained glass block until the stained glass is removed, and replaced with a solid block. If the glass is removed, the light will still be there.
Linked issues
Attachments
Comments 2
Duplicate of MC-37374
cannot confirm make sure you aren't using MCEdit who saves and recalculates light in the chunks as it still thinks it's a locked chest