From what we can tell of the sketch, it seems to show off a dual-camera setup. Does this seem like a stretch? We suppose in some ways it could be, but recently leaked photos have shown off the back of the phone which seemingly confirms the dual-camera setup. Either that or we could be looking at a fingerprint sensor.
The photo and the sketch are a bit hard to see so like we said, it is possible that it might be a dual-camera or a fingerprint sensor. However its design is reminiscent of the HTC One M8 which had its dual-camera setup the same way, which is why we have an inkling that it could be that. Then again this is just a sketch and not the real deal, so for all we know it could have simply be an early design of the phone.
In the meantime OnePlus has confirmed several features of the phone, including the use of the Qualcomm Snapdragon 810 processor, a fingerprint sensor, and USB Type-C connectivity, but until we hear more do take it with a grain of salt for now.