I looked at your code and know exactly why your legend is doing this.
Early on in your code, you used this command four times before anything
These are your four buildings, and you made them with black lines. When you use the legend command, it automatically generates the color and style in the same order that you plotted in . This means your first for legend entries you make will be these black lines.
There are different ways to solve this problem. I recommend saving handles of the plots you have. Robert has shown you how to do this, I'm just identifying what the problem was.
If possible, it might be easier just to plot your data first and the buildings and red triangles afterwards... this will give priority to your real data to be made in the legend first! :)