Final answer:
A small-caliber gunshot wound to the back with minimal bleeding would most likely be discovered during the detailed secondary assessment, where a thorough examination of the body is performed.
Step-by-step explanation:
A rapid head-to-toe assessment is usually a quick visual and physical evaluation that allows for the identification of obvious major injuries or conditions. For a small-caliber gunshot wound to the back with minimal bleeding, such an injury may not be immediately noticeable during this rapid assessment, especially if there's clothing or if the patient is lying on their back. In contrast, the detailed secondary assessment, which involves a more thorough examination of the entire body including removing clothing and palpating for wounds or abnormalities, is more likely to reveal such a hidden wound. During the primary assessment, the focus is on immediate life-threatening conditions like airway obstruction or severe bleeding, which may cause a small-caliber wound to be missed if it is not actively bleeding profusely. Additionally, the general impression gathers critical information at a glance, prioritizing life threats but may overlook less obvious wounds. Imaging techniques such as MRI or CT scanning could localize the injury, but they are not typically part of pre-hospital care. Therefore, the detailed secondary assessment is most likely the phase where such an injury would be discovered.