Sketches are much more than marks on paper; they play a key role for designers both in ideation and problem-solving as well as in communication with other designers. Thus, the act of sketching is often enriched with annotations, references, and physical actions, such as gestures or speech - all of which constitute meta-data about the designer's reasoning. Conventional paper-based design notebooks cannot capture this rich meta-data, but digital design notebooks can. To understand what data to capture, we conducted an observational study of design practitioners where they individually explore design solutions for a set of problems. We identified sketching and non-sketching actions that reflect their exploration of the design space. We then categorized the captured meta-data and mapped observed physical actions to design intent. These findings inform the creation of future digital design notebooks that can better capture designers' reasoning during sketching.