If it's honey fungus then when you dig and find the white mycelium it will smell strongly of mushrooms. We have honey fungus in our village and it is a terrible thing - plants just die back and in a year they are dead. I have never seen the black "shoelace" threads that they are supposed to produce though. The toadstools appear after the infection has taken hold.
Watering with jeyes fluid or similar may help to check the disease if you find any.
Have you seen any other dead trees or shrubs in the area.? We had large numbers of elm which were lost to dutch elm and the honey fungus seems to have started on all the old uncleared stumps.