If you take a look at the RS web site you will see a whole range of SMPSs that comply with IEC 60601, which relates to the earth leakage current. A voltstick is only an indication, some of them being triggered by magnetic as well as electrostatic fields.
If the voltstick lights up on the case of a plastic enclosure it could be just because there is a live track on a pcb just inside the enclosure. If on the other hand it lights up when you touch the body on the output plug, that would indicate that there is more than just a couple of volts on the plug. THe final proof is always to measure the leakage current with a meter.
