Because of the way sketches are used in the process, they are very often vague and incomplete in nature. Also, the same object can be represented in completely different manner in a sketch depending on its relevance. For insance these sketches all represent similar trees, but with varying amont of accuracy.

Design process is not tied to time and place, and most architects and designers have their mind continuosly working with the ideas, at least on subconcious level. This means that sketching may happen anywhere. You have probably heard tales of great buildings being first designed on back of a cocktail napkin or restaurant place mat. This is no surprise actually, sketching is often most productive in a relaxed environment, outside the design office, with good food and good company.