Estimated QPE appearance during observations with no clear QPEs (XMM2 and XMM7 to XMM11). Shown is the relative 0.2-2 keV QPE amplitude as a function of the ratio of QPE to quiescent temperatures for the XMM6 and XMM12 observations. A_rel and kT_QPE/kT_quiesc have been rescaled to show how additive QPEs would have appeared during high-luminosity observations with no unambiguous QPEs (see text for details). The dotted line shows a simple fit to the data of the same form as that used for Fig. C.1.