I've seen this on several boats with PSS shaft seals... You can't feel it, you can't seem to "see" it...but there is a line of corrosion (like laser level made it) normal to the shaft.

Several people have said to me, that's the reason. I installed a seal, and we don't have it. Other boats that didn't, seem to exhibit it.

I had the same reaaction. IF You don't have any metal which is that line, then don't worry about it.