Which item would likely not be considered essential for keeping field notes?

The choice of a full-sized notebook as something that would likely not be considered essential for keeping field notes is valid because field notes can often be effectively recorded in smaller, more portable formats. Many security personnel prefer using smaller notepads or memo pads that are easy to carry and can be quickly accessed when needed.

On the other hand, having two pens ensures that there is a backup if one fails, making it a practical item. A tape measure is useful for documenting measurements related to security assessments, and a watch helps in keeping track of time during shifts and incidents. Therefore, while a full-sized notebook may be useful, it is not as essential as those other items for the immediate and practical needs of a security guard in the field.