I had to laugh when I read the title to this thread. I actually call one of my alternate routes "the secret path." It's a short dirt and rock trail through a woods at the back of a park, then through a deserted railroad siding and a small factory loading zone and parking lot. It saves about a half mile on my commute to work. I found it by exploring on my own, then looking on Google Maps for some ideas.
I have many routes that I use to get to different parts of the city. My criteria are that they be scenic, low-stress, faster and/or shorter routes. My inspiration is little kids. They are always exploring for new routes that nobody knows about. Just keep looking "farther and deeper" and you'll find some great short couts.