Possibly is a rat. You'll have to keep a look out & hope that you spot it. I know of someone who had rats breeding under their decking which of course they had to deal with. They also had to stop feeding the birds as the rats were seen running up the pole to get at the food, it was a metal pole to, you would expect them to slip but they didn't. You are lucky to have hedge hogs. I haven't seen one for years.:-(

It could also be mice - they love a bit of decking to nest beneath. I suppose voles might be a possiblity too...

also squirrels will dig small holes too. how much gap is there between the decking and the soil.