Which of the following is a common indicator that may signal a possible terrorist threat?

Unusual behavior, such as someone taking photos of critical infrastructure, is recognized as a common indicator that may indicate a potential terrorist threat. This type of behavior may signal that an individual is gathering information for malicious purposes, particularly if the photographs focus on sensitive or secure locations that are typically not of interest to the general public. Such actions can highlight unusual levels of interest in targets that could potentially be exploited for an attack.

Other options, like routine inspections of machinery, demonstrate normal operational procedures and do not inherently suggest a threat. Increased community gatherings can be simply benign social activities and do not necessarily correlate with terrorist intentions. Visible security personnel generally serve to reassure the public and deter threats rather than indicate the presence of such threats. Thus, recognizing unusual behaviors tends to be a more direct indicator of potential risks.