W1 − W2 colour of the galaxies in the sample, plotted against ΔL_W2. The vertical dashed line shows the luminosity criterion that we used to detect transients in our sample. A commonly used criterion to detect the presence of an AGN is if W1 − W2 >  0.8 (Stern et al. 2012), shown here by the horizontal line. For the five transients, the colour shown is derived from the adopted host contribution, as described in Sect. 3.