What is the cause of the stain?

Hello!

I need help…i just noticed that i got this stain on my sink when i was cleaning the cabinet under my kitchen sink…i am showing a photo of the part of my under mount stainless steel sink from Amerisink.

I want to know what is the cause of the problem ibefore i call a plumber. is there a leak on my kitchen faucet? or maybe its a defectice sink? or it was not sealed properly when it was installed.

i would appreciate it if someone can tell me what the problem is.

Thank You!