Once again an NH3 guy in trouble!

This plant has a 25,000 square foot processing area doing fruits and vegetables. We have a total of twelve upblast coils, Krack BTRC-316-DXF. (Copper) Refrigerant is 404a at forty-seven lbs suction. Room temp is thirty-six to-thirty-eight F. Three in a finished product cooler, three in a raw product cooler, and six in a process area. The coils in the FPC were replaced last year due to corrosion on the fins. Two others were replaced in production because of the epoxy electrofin coating coming off. None have been known to leak before.
Last week we had a leak which we traced down to a coil in production. After a long search we found a leak in a bottom tube. We lost three hundred of thirteen hundred pounds.
We valued the coil off and removed it this morning. When we got it down we discovered two leaks. The second was much worse than the first. I do not understand how we could have missed it.
To my knowledge none of the coils has leaked before. This one had two places where very small spots had corroded. Just a little crater with a pinhole at the bottom.
These two holes were on two tubes on the same circuit parallel to each other, three feet apart.
Because of the fins we cannot see the tubes but I see little bits of green in between the fins. We have repaired the tubes and will let it sit under pressure over the weekend.
I am surprised that both leaks occurred close together in the same circuit and at the same time. Could a stray electrical current be causing this? Any ideas would be welcome.