Surreptitiousness is the noun for the quality of being surreptitious — the secretiveness itself, the sneaky manner. Not the action, but the hidden-ness of the way it was done: "There was a surreptitiousness about the way he entered the room." "The surreptitiousness of the deal made everyone suspicious." Be honest with your students about this one: it is a real word and dictionaries list it, but it is genuinely rare. Native speakers usually reach for "secrecy" or "secretiveness", or they just rewrite the sentence with the adverb — "the way he surreptitiously entered" sounds far more natural than "his surreptitiousness in entering". So learn it to recognise it if you meet it in a novel or an essay, but you do not need to use it. It carries a slight flavour of judgement: calling someone's manner surreptitiousness suggests you find it a bit shifty.