This paper tackles the problem of re-localization in a swarm of moving virtual robots where some robots might be lost and where the distance between each neighboring robot is known within a limited communication radius (e.g. using the strength of a radio/wifi signal), whereas the precise localization is not known. The basic process in relocalization relies in a robot swarm that re-aggregates based on local information only by recruiting lost robots so as to build a swarm configuration where localization is possible and non-ambiguous. The main issues in relocalization is to provide highly decentralised behaviors for ensuring efficient and fast re-aggregation of the swarm that is scale independent. In this paper, the problem of relocalization is defined as well as criteria for evaluating swarm relocalization efficiency. Moreover, a set of decentralized behaviors based on local reactive behaviors is presented and experimentally studied.