Surprise Attack within Party

[Joe the Rat]

From your description of the rogue's methods, you should be rolling Deception vs Insight instead.
The surprise is not the rogue's presence, but their intentions.

Seconding on this - it's detecting the motive. Note you should also look at this initially - passive insight, unless someone questions the motive.

My other thought is Perception vs Sleight of Hand (concealing the move), but that's basically one Dex check or another.