My job moved into a new office building. Inside there is a water cooler. Are we allowed to use the hot water part of the if we are not sure if anyone not Jewish used it before us?
You my use the cooler even for hot water, because even if it was used for non-kosher, it would only be an issue of nitzuk, and an aino ben yomo, which is permitted in retrospect. There shouldn’t be an issue of zeiah here because the steam rising from any cup would be only kli sheini and the outside of the machine is cool. Besides, in you case you aren’t even sure if it was used for non-kosher.