More and more mobile devices feature wireless communica- tion capabilities. They can self-organize in a mobile ad hoc network in order to communicate and maintain connectivity without any infrastruc- ture component. In this context, some devices may benefit from established trust relations in order to communicate private data. Various solutions already exist for establishing and detecting such trust relations. But is it still possible to detect a trust relation in an unobservable manner? That is, in a way that an attacker cannot understand whether devices share a trust relation or not. We exhibit a solution to this problem. Our solution guaranties the anony- mity and the unobservability of participants against passive and active attackers. The security properties of the solution are machine checked with the AVISPA framework [2] and the SPAN tool [5]. The main applications could be found in mobile ad hoc networks and in vehicular networks [6, 7] where anonymity and unobservability contribute to a better privacy.