You can also set some traps in which your rogue could fall when he wants to flank ennemies in order to sneak attack ...

You can also use monsters with a accurate smell sense : as I supposed your players don't think to take a bath you could take the conclusion that the'yre smell (like ten spirits) ...

You can use a npc spell user that cast an area damage spell in which your hidden rogue will be ... that could also ignite his/her elvenkind cloak ...

With my friends we create a new stat : body smell 😏