The origin of the iron fluorescent line at 6.4 keV from an extended region surrounding the Arches cluster is debated and the non-variability of this emission up to 2009 has favoured the low-energy cosmic ray origin over a possible irradiation by hard X-rays. By probing the variability of the Arches cloud non-thermal emission in the most recent years, including a deep observation in 2012, we intend to discriminate between the two competing scenarios. We perform a spectral fit of XMM–Newton observations collected from 2000 to 2013 in order to build the Arches cloud light curve corresponding to both the neutral Fe Kα line and the X-ray continuum emissions. We reveal a 30 per cent flux drop in 2012, detected with more than 4σ significance for both components. This implies that a large fraction of the studied non-thermal emission is due to the reflection of an X-ray transient source.