Encountering cryptogenic populations that are either native or introduced is a common but underreported phenomenon in field biology. Such local species' occurrences of unknown origin hamper our understanding of species' natural distribution ranges, and pose a problem to conservation management decisions. Genetic tools are frequently used to infer the ancestry of natural or invasive populations based on spatial geographical variation. Here we describe the occurrence of cryptogenic crested newts Triturus cristatus c. 25 km away from their main range and situated in an area where an introduction has taken place half a century ago. We first verify the suitability of a coalescent-based analysis that reconstructs the number of founder individuals of a putative propagule from a source pool, based on previously published data documenting a well-known introduction (the Laysan finch on the Hawaiian archipelago). After showing the validity of the approach, we apply this analysis to the case of T. cristatus, revealing that the number of effective founders would have been of the same order as the current effective carrying capacity of the population. We thus argue that the local T. cristatus occurrence is in fact of natural origin, with the documented introduction having little or no impact to the current gene pool. The result has major implications to our understanding of the species' habitat requirements in a zone of parapatry with a closely related species.