I remember a number of years ago doing some plumbing work at my grandmas house- I was replacing a section of galvanized pipe with copper. I had remove the clamp for the GEC. (When that service was installed it was still up to code to connect near the panel and rely on the piping as long as you still had the meter jumped out). I put my clamp on ammeter and it was drawing 10 amps. It did draw an arc when I disconnected it, but checking with a dmm, there was no difference in potential. That told me that one of the neighbors neutrals is probably compromised. Much more recent we were connecting a new GEC to the water main for a 600A service upgrade in a downtown building, and witnessed quite a bit of arcing upon disconnection of the old GEC. Still no difference in voltage.
One should really tread lightly in these instances, some or most of that current could be the utilities neutral current due to parallel paths and marginal connections.
