One of the sysadmins at a friendly company had the same problem... only that it was a >2km link in a densely built-up city and it turned out to be a crane that

One of the sysadmins at a friendly company had the same problem... only that it was a >2km link in a densely built-up city and it turned out to be a crane that moved in on rails each morning with the beginning of a shift and then moved out in the evening! Took a guy standing there with binoculars to realize what was going on...