unwanted dashed line under object

I'm just learning. I bought a textbook and I'm working through the exercises.

I'm stuck on creating a small fold in an object. I need to make a triange, and position it underneath the object. This makes it look like it's got a fold. 

The triangle is created with the pen tool, no fill and black stroke. I expect it to disappear when I send it back (under the object). But it doesn't. It's still visible, but now it's a dashed line. Dashed line is not selected. It does go under the object, because I can't get hold of it or move it. 

I can't find anything online that will explain what's happening, or how to correct the issue. Any help would be really appreciated. Thanks!