The Roomba finds it's charging base like this:
The charger has a beacon (it emits light)
On top of the roomba there is that donut-shaped lens that funnels light from all direction, downward to an infrared sensor. When the roomba can sense the beacon signal with this, it starts to rotate, as there is a second infrared receiver on it's side (over the charging terminals) and with that, it homes in on the charging base.
// Per.