Party of Three

Find the hidden Companion Cube tucked away in a co-op chamber. It's in the final area of the Excursion Funnels course (Course 4): shoot a portal at an out-of-the-way surface off to the left and ride the funnel across to reach the cube behind a set of opening doors. Go and give it a hug while you're there. Easy to grab on the same careful run as Still Alive.