Nickel Contamination of Gold Bath without a known source

Q. PPM Nickel Increase in Gold Bath?
Hello everyone. This is an acidic, nickel-hardened, gold bath. The mystery lies in the fact that I have not added any nickel since August, to control a sudden increase in hardness values. The nickel values have fallen back down since then, but not by much. I even removed 4L of solution at the beginning of the month, only to receive a value almost identical to the month before (in fact, a little higher).

We can omit drag-in (rack plate only, one specific part only, and coming from an acid rinse not a nickel bath or a nickel contaminated gold strike, and equipment, I believe). This bath also sits at the end of the process line, so nothing is dripping into it. I'm currently working under the assumption that we may be 'scrubbing' nickel into the bath from our high current densities (10-12 ASF). Without getting into details, this is the optimum current density for this specialty part. Am I missing anything? Am I overlooking something?