The paper tackles the problem of identifying an individual transfer function in a network of linear dynamical systems in the presence of loops under the assumptions that (i) only a subset of the nodes is observable, and (ii) data are being passively recorded (i.e. it is not possible to intervene on any part of the system by actively injecting an input). Such a scenario is often encountered in the study of many naturally occurring systems and is also motivated by operating networked systems where the injection of an external signal might lead to undesired disruptions. Sufficient conditions on which signals should be observable to guarantee the identifiability of a desired link are provided. The results generalize well-established identification criteria developed in the context of graphical models.