Use of "en"

Question

I'm not sure this is directly relevant to this topic but I can't think of a better place to ask the question!

The answer to a question relating to this lesson is "Je vais me faufiler sans qu'il s'en aperçoive." What is the function of "en" in the sentence (and the rule pertaining to its use)? I assume it intends to refer back to the previously mentioned sneaking, but I can't figure out the rule. I know that "en" can be used to refer to previously mentioned phrases starting with "de", but that doesn't seem to fit here.

Any help would be greatly appreciated!

Accepted Answer

Hi Tom, 
If you didn't use 'en' you would have to say -
"....sans qu'il s'aperçoive de ça "
so it replaces 'de ça' =  of it 
Hope this helps!

Answer

S'apercevoir de quelque chose. -- To be aware of something, to notice something.The adverbial pronoun en refers back to je vais me faufiler, as you correctly assumed. You need en to refer back because the phrase s'apercevoir de quelque chose introduces the thing you notice with the preposition de.