We have prompted our vendor supplying us the Killark junction box and below information in blue fonts provided for your information as you are schedule to meet CUEL representative for joint investigation to the rusty stains on coming Monday, 8th Feb 16.
In overall, look like condensation is a big part to play as junction box cover are properly protected as well as thread sealant for the SOV connection at the top of the junction box.
Hope below information of help to you.
Awaits your findings advice and try to inspect as many as possible for a more conclusive summary.