mystery nest

Observation - mystery nest - UK and Ireland. Description: when checking woodcrete nest boxes (great tit sized hole) I found one which had simply been loosely packed with earth up to the level of the access hole. I've looked up the nests of any birds that I thought could have been possible occupants but nothing

when checking woodcrete nest boxes (great tit sized hole) I found one which had simply been loosely packed with earth up to the level of the access hole. I've looked up the nests of any birds that I thought could have been possible occupants but nothing fits.
Any ideas? thanks!