Brown Silicone Stain on DSA

[epon]

Found this brown spot in the overflow box silicone, passed the fingernail scratch test and doesn't appear to be leaking ... any ideas whats going on here?

 

[steallife904]

don't see a pic?

 

[epon]

pic is linked to flickr image so it probably just didnt load, maybe refresh the page

 

[jalisco]

I can see the pic and have the same discolortion on a used Oeanic Tech tank I'm about to fill.