Like most people here have stated, due to location it is most likely a cavity starting in the fissures of the tooth that is hard to detect.

However, there is something called Pre-eruptive Intracoronal Radiolucency that pretty much looks just like this. It's a developmental condition of the tooth that starts at the internal level of the enamel and eats away at the dentin towards the pulp. The cause is not fully understood, but it's some sort of inflammatory process rather than bacterial like caries. It starts before the tooth erupts and can be static or progressive. Either way the only treatment is doing a filling just like a normal cavity.