Well, if it can't be depressed, then it could be a bit of trapped debris or even the start of a plating blister, where the zinc core is starting to deteriorate and expand. Either way, it would be a spender in my book. On the other hand, the cost of ownership is low so keep it if you like it!
"If you climb a good tree, you get a push."
-----Ghanaian proverb
"The danger we all now face is distinguishing between what is authentic and what is performed."
-----King Adz