It was often not advised to place the trap below a hill,because if the mole has to shift and remove soil,from the tunnel,its going to be around the vicinity of the hill.Think of where the shaft to the surface , will be ? But as others have expressed, its not impossible to catch there,....
Sorry, should have finished by saying, this increases the chance of the trap being sprung, by the soil & stones, etc that the mole may push in front , before he is in it ! lol