Hi! Was wondering if anyone could help..

I make real gold plated sterling silver pendant necklaces. I had one customer email me a few weeks ago saying they had been on holiday for a few weeks and just got back to their package and when they opened the box the pendant had brown spots on it. They sent me a photo and it didn't look like rust or anything it looked more like a chemical reaction or something like that.

Now today I just got a package sent back to me as it was undelivered/unclaimed and that necklace also had the brown spots on it. I posted that last month so its been sitting in the box for a while since it was sent overseas then sent back to me as it couldn't be delivered.

I have made lots of necklaces and I have never had this issue other than these two times - and the two things they both had in common was they were both left in the box for a long period of time. I package all my necklaces in a jewellery box with a velvet insert.

I wiped the pendant with a polishing cloth and they came off straight away so I'm wondering if anyone has any idea what this could be? And how I can prevent this happening?