Electronic sensor of some kind but what?
It might be a sonic mole repeller.  It looks like a cap is missing from one end.  That end (shown in the third picture) has what looks like a piezoelectric transducer that is used to make sound to repel the moles.  Perhaps they mounted it in the pipe to make it easier to change the batteries.



(From http://www.victorpest.com/sweeneys-solar-power-sonic-spike-s9014 )