I believe Rovio will have an object sensor system as well, to navigate around surprise obstacles. As far as going under an object that obscures the "gps" vision; I imagine it's just going to operate on the assumption it will eventually wander out of the blind spot and back into the light.

I've seen other bots that, when something obscures locating the charging station, will sound some kind of alarm to let their person know they need help. I wonder, since it's supposed to be wifi-ed, if it'll send an email?

From: Rovio\n
Subject: Help!\n\n
I'm somewhere where I don't know where I am!

All of this is pure speculation on my part :) 

Watch out, don't step in the anthropomorphization.