Camel - I should think that depends on size and design. (Although the old C-133 could be detected and ID'd at any altitude at >10 miles, WUUUmmm-WUUUmmm-WUUUmmm.)
And while I hate to play the SUMMONS LEON JABACHJABICZ card, if you want something quiet that can patrol at speeds compatible with a boomer sneaking out to the ocean...