Electron cloud (e-cloud) is one of the major limitations for beam quality in modern particle accelerators. The macroscopic property which governs this phenomenon is the secondary electron yield (SEY) of a surface, defined as the number of emitted electrons per incident electron. SEY of inner surface walls must be less than 1.3 to prevent the formation of an e-cloud. Although most pure metals possess an SEY within this range, technical surfaces (i.e. those resulting from the necessary machining to produce vacuum parts) typically display much higher SEY. An elegant and effective solution to this problem is to deposit the carbon coating on these surfaces by magnetron sputtering. However, the first measurements performed at CERN revealed an increase of the SEY as a function of long term air exposure. Furthermore, we observed a rapid increase of the SEY of these samples whilst under high vacuum conditions. In order to determine the contaminant responsible for the observed ageing, as well as the ageing mechanism, the samples were exposed to various gases and vapours such as water vapour, H2, rotary pump oil vapour, etc. The results confirm that the vapour of rotary pump oils is responsible for exceptionally fast sample ageing. We also observed that high SEY samples usually have an increased surface concentration of oxygen. The possible ageing mechanisms are discussed.