A nice simple way to pop out the silhouette of a cave entrance or other level detail is to use a silhouette card to fill the area with a tint of light. With some depth fade it gives the impression of a localised fog that fades away as you approach - great for helping to highlight an area in a controllable and stylised way.
I first heard about it in this wonderful talk about the Art of Abzu https://www.youtube.com/watch?v=l9NX06mvp2E (the "portal cards" are shown off at 18 minutes in)
UPDATE: I've made a new (better) version of the shader - please download and use Silhouette Light Plane 2.shader instead. It's no longer additive, because I think it's more useful to be able to control that yourself. I've also added in a texture mask, for more control over the falloff if you don't wish to use the edge fading.
I've also fixed a bug by clamping the Edge Fade amount, which was causing negative colour values. To fix the old version (Silhouette Light Plane.shader, which is included in all unitypackage and zip files), simply Add a "Saturate" node to the output of the "Edge Falloff" section, before it pipes into the final emission colour multiplication.