Mashgichim often have to ‘work’ on Shabbos and Yom Tov. This of course does not refer to doing any actual ‘Work’ that is a melacha, but merely to supervise.
Specifically, there are areas of kashrus that require the presence of a Jewish mashgiach by every shift, such as milking cows on a dairy farm, and the delivery/pressing of grapes in a winery (up until the point where it is pasteurized and becomes mevushal.)
Even in other areas where the mashgiach must be ‘Yotzi Venechnas’, he must have the possibility to enter for a surprise inspection at any time. Therefore they sometimes stay in a nearby hotel and pop in for a visit on Shabbos too.