It has been shown in 2005 by neutron powder diffraction that the triangular layer Heisenberg compound NiGa2S4 is characterized by nano-scale magnetic correlations below ~ 18 K. Using the muon-spin rotation and relaxation (μSR) techniques, we have evidenced that the compound displays a phase transition at Tc = 9.2 (2) K below which a coherent oscillation of the muon spins is detected. Remarkably, the magnetic specific heat Cm exhibits a rounded low- temperature maximum at ~ 13 K, i.e. at a temperature above Tc. This is consistent with the theory of Kawamura and Miyashita which attributes the transition at Tc to the unbinding of Z2 vortices. NiGa2S4 is characterized by a wide spectrum of magnetic fluctuations as inferred, for example, from the difference in the temperature scale deduced from μSR and neutron scattering.