C.T.
My son has the same thing, and yes, she has most likely worn her teeth down off each other. I noticed a chip in my son's tooth when he was 2, and took him to the dentist. Like your daughter, there had been no injury or trauma that I was aware of. The dentist said that it was from his teeth wearing against each other (and pointed out a much smaller indentation on the bottom). I was surprised because it seemed to come on suddenly and the 'chip' seemed relatively large (to my eyes -- the doctor was hesitant to call it a chip!). She said it had probably been wearing away for sometime, but maybe a largish piece broke off recently (just during eating) which is why I noticed it.
She said we didn't need to do anything about it, but she would monitor his teeth during his regular check-ups. My son has a slight overbite from sucking his thumb when he was younger, so that could be why his teeth are wearing away (my older son's teeth have not done that). He's now 3, and that's the only largish chip missing, but I see some signs of wear on other teeth, but nothing that concerns the dentist.
So, yes, take her in to the dentist, but its probably not an emergency.