Actually, do we have to roll for every single little thing? I'm a follower of the idea that PCs should roll only in the situations where a failure carries the risk of harming either the character or its plans. Not noticing Esther isn't going to harm you (I hope so, for the time being), so the best thing to do is to roleplay it. If your Perception score is at least superior, awareness bonus included (seen the chart in the chapter 1st?), you will fail to notice the assassin sneaking upon you.
Also, I want to oficially greet you in our little crew, Esther. If you really enjoy backstabbing, I guess we'll be going along just fine... some of us do it for a living, you know. ;)