A lot of the “cone of sight tutorials” I’ve seen visualize the sight ray-cast cone on a 2d level, but never on a 3d level.
I was wandering how to get that cone of sight that most top down stealth games have, except in 3d. So a 3d cone.
One solution would be to import a model of a 3D cone and use the OnCollision methods.
You could use a method like the one explained in this blog post.
But instead of casting in all directions, you would cast in the direction the player/entity is facing. And you would need to do a 3D grid of rays in the shape of a circle/cone.
Then you could use the point-of-collision of the cone rays to procedurally modify a cone mesh. Jayelinda has some good procedural mesh generation tutorials, this one covers cones. However, she doesn’t generate cones in the way that you would need, so you’d have to figure that out. But anything worth doing should be challenging, right? 
If you combine those two methods, and some elbow grease, you should be able to put together a vision cone in the manner you described. It also might be possible to do this with shaders, but I’m not personally knowledgeable enough in that area (yet) to give advice.
Edit: I feel like it should be possible to use ray marching to achieve this effect, but I’m not sure how to do it exactly.