That scandal isn't something we can keep under wraps forever. Someone's going to sniff it out eventually.
Literal
That [of-の] scandal [topic-は] that-forever smelly-thing on-lid [quotative-と-いうわけ]-ない probably. Eventually people [topic-は] will-sniff-it-out.
Two notable idioms. 臭いものにフタ ('cover the smelly thing with a lid') is a 四字熟語-style proverb meaning 'sweep problems under the rug.' いうわけにはいくまい uses the classical negative-volitional まい ('surely will not') on the pattern いうわけにはいかない ('cannot be said to be').「嗅ぎつける」 ('sniff out, catch wind of') is a compound verb.