Could easily be completely occluded, it's just bad practice to not give some indication of it existing in drawing. Not because it's incorrect, just 'cause it detracts from the aesthetic of the final work (Assuming the purpose is to beautiful/aesthetic, like this picture definitely is)
This might just be a study with regards to a reference and it's common for reference imagery to have elements that make it look weird as a drawing. Like, overlapping parallel edge lines is one of those things you should generally try to avoid.
