Old bluebird box

After watching the bluebirds (Mr. and Mrs.) fly in and out of this box for a few days, I opened it up and was surprised to see it was totally empty.  I thought they were starting a new nest.  This would be the third year in a row that we have had bluebirds nesting in this box.  I was disappointed.The is the old bluebird box, lived in and weathered.
This is the entrance of the birdhouse.  We're pretty sure it's the woodpecker who's been at the doorway.