Brown Lines/ Spots in The Grooves of my Back Molars?

On Tuesday I was shining the flash light on my mobile phone in my mouth ( as you do lol) and I noticed this little brown line in the groove on my very back molar on one side and a couple of tiny, pinprick brown spots on the other side. My teeth haven't looked any different than before and you can only see the brown stuff when I shine a bright light in my mouth. They don't hurt or feel sensitive. Its quite a faint brown colour. I had the 2 molars in front of these taken out in march for my brace and nothing was mentioned about it then. I had an x ray at the end of february in that area on both sides of my mouth and there was no sign of decay then. Also I went to the orthodontist on wednesday he he was attaching the wire to the very back teeth and didn't say anything about it. I brush my teeth for three minutes twice a day and use cavity protection mouthwash. Is it possible that they are cavities?