From Wiktionary, the free dictionary
à plein nez
- (figuratively) a lot, clearly, strongly, all over
- sentir à plein nez ― to reek of, to smack of, to have a strong whiff of
- ça sent l’arnaque à plein nez ― I smell a rat, there's something fishy
Always used with the verbs sentir (“to smell”) or puer (“to stink”).