Final answer:
Indications and warnings of terrorist activities against Navy installations predominantly come from Navy intelligence agencies. They pool and scrutinize information to detect potential threats.
Step-by-step explanation:
Indications and warnings of terrorist activities against Navy installations are most reliably received from Navy intelligence agencies. These agencies are tasked with the collection, analysis, and dissemination of information related to global threats, including potential terrorist acts. They monitor various channels of information, which may sometimes include tips from local community groups, social media platforms, or foreign diplomatic channels; however, the primary and most structured warnings come from the dedicated intelligence apparatus serving the Navy's security needs.