If they have craft that can travel between solar systems then our military technology is not a threat. Besides, why make your presence known when you can just hide deeper from everybody? People tend to notice with multi-million dollar craft manned by people with families suddenly go missing, especially in American waters at peacetime with modern geolocation and communications technologies installed.

If fully manned Navy frigates and destroyers were vanishing without warning or explanation there is zero chance of keeping a lid on that. Way too many people involved, many of which are civilians.