Failure detectors are classical mechanisms which provide information about process failures and can help systems to cope with the high dynamics of self-organizing, unstructured and mobile wireless networks. Unreliable failure detectors of class ◊S are of special interest because they meet the weakest assumptions able to solve fundamental problems on the design of dependable systems. Unfortunately, a negative result states that no failure detector of that class can be implemented in a network of an unknown membership; but full membership knowledge as well as fully communication connectivity are no longer appropriate assumptions to the new scenario of dynamic networks. In this paper, we provide a discussion about the conditions and model able to implement failure detectors in dynamic networks and define a new class, namely ◊SM, which adapts the properties of the ◊S class to a dynamic network with an unknown membership.