Never noticed if it existed under Marshmallow, but on Nougat I did spot the developer options is easily visible without the need to search for some hidden menu.
Phones are always shipped with developer options hidden because it's a feature for power users. I don't have android N but I've seen this blog with how to enable said menu in settings. Someone must have enabled it on your phone.
This can be disabled as well, once you're done with it...
Let me go through it but I hardly give out my device so I don't think there is anyone who might have accidentally made it visible. Unless it's me may be. So I can't rule that out. I should hide it then.
Will check this out later
I don't think there's a need to hide it because nothing there can break your phone.
Then that's cool.