It seems that you're seeing my daughter en stoemelings!
The useful adverbial phrase, which is very current in our region, en stoemeling(s), corresponds to the French "en tapinois", meaning furtively (from the Old French "a tapin", in secret, from "se tapir", meaning to conceal yourself): in secret, on the sly, underhandedly, cunningly, concealed, incognito or even clandestinely, secretly, sneakily.
As you can see, this phrase is used to express several notions which are all quite similar.
Stoemeling comes from the Brabant "stoem": silly, naïve, stupid; it can also be found in the regional and old Dutch "stommelings": without speaking, without words, and in "stom": mute.
Despite this origin, the phrase is in fashion both in the North and the South of the country. There is even a micro-brewery in Brussels called "En stoemelings" which produces a beer of the same name. King Philippe paid a visit recently, but he didn't do it en stoemelings because the press had been called in to cover the event!