going to the dentist isn't even a sure way...really don't kid yourself...
I had a tooth that had a "darkish" spot, but they never knew what it was, wasn't caused by cavities I guess...just they way my teeth are.
I had a tooth that always had this deep crevice in the side, it was always dark...never a cavity but every time, they'd try and wiggle that little hook in there to see...lol
but maybe you do have some micro stuff going on...buy some of that enamel reforming toothpaste, not sure it works but eh...