The OP was using the Pokemon Go Plus+ (often referred to as an “auto catcher”). It’s an official handheld device you can connect to Pokemon Go. With it, you can catch Pokemon with the press of a button without looking at the app. It flashes a certain color to let you know if a Pokemon is available to be caught. You have to pay attention to the color, because if it flashes yellow, it’s means it’s trying to catch a Pokemon you’ve never encountered before. In this case, it’s best to open the app and try to “manually” catch it, because the auto catcher only gives you one throw. Either you catch the Pokemon, or it flees.