Do I need to take any ESD precautions when I service or repair products in the field?
The challenge that you face in field service is that you are working in someone else's environment, one over which you have little control. You may face low relative humidity, or an office with static generating carpet, or a lack of work space. Your customer may not know whether the products you work on are ESD sensitive or not.
You need to be adaptable to the environments in which you are working. For example, in addition to your wrist strap, carry extra alligator clips, jacks, and extension cords.
You should assume that all the products you work on are ESD sensitive and handle them accordingly. Take the extra time to clear a work area large enough to place your protective work surface.
Follow good ESD protection procedures at all times. Your responsibility is to help solve your customers' problems, not contribute to them.