When you lock eyes with a stranger of the opposite gender, is that always a good sign?

Depends on their eyes. If they're squinted, and they give you that sudden feeling to look away, it's usually bad. Yesterday, I was with my cousins, and I locked eyes with this beautiful girl, while I was smiling, and it was coincidence she was smiling too. We locked eyes for about 3 seconds(we were still smiling) and then I looked away. Not because I was intimidated or was a coward, but because I knew she was gonna check me out after I turned my back on her... and because I didn't want to ask for her number or flirt a bit. Not to mention I was with my cousins and we had to go. She DID check me out after I continued walking past her. So yes, eye contact for more than 2 seconds is usually good if they have that flirty glint in them.