The Boker Plus Urban Survival is a thin pen-like blade. It used to come with both a glass breaker and a pen clicker end piece. Don't know if it still includes both. I'm not sure I would want anything bigger clipped inside a polo. But I think it was designed for exactly what the OP described. And comes in Camo, in case you want it to disappear on your shirt.

Below is the description from the Manufacturers site....
"Our popular Urban Survival mobel by Jim Wagner gets a new look! With an overall length of 5 3/4" and a blade length of 1 5/8", the Urban Survival Camo offers the ultimate in carrying comfort with its compact dimensions. The sleek construction and 440C stainless steel, scalpel-like blade offer optimum control. Durable aluminum handle, now with a camo finish. Includes pocket clip, blade lifter, and interchangeable glass breaker and false writing pen tip. Weight: 1.5 oz."