Modern distributed systems deployed over wireless ad-hoc networks are inherently dynamic and the issue of designing dependable services which can cope with the high dynamics of these systems is a challenge. Byzantine failure detectors provide an elegant abstraction for implementing Byzantine fault tolerance. However, very few works have been proposed for the new distributed system scenario. This paper presents a model and a protocol able to implement an unreliable Byzantine failure detector adequate for dynamic networks with unknown membership. The protocol has the interesting feature to be time-free, that is, it does not rely on timers to detect omission failures. To the best of our knowledge, the adoption of a time-free Byzantine failure detection is novel and this paper provides a first insight towards the understanding and implementation of such an approach in networks with unknown membership.