Yes the BCU does all sorts of things that the ECU does not. Whilst not an accurate reflection, but as a guide, the fuses are split into two banks on the recent +4 Block A and Block B
Block A are things associated with the BCU and Block B the ECU

So the BCU gets involved with the immobiliser, lights, wipers, heated screen etc. It will always be drawing power for the immobiliser and maybe other functions.

You will have noticed that there are only three traditional relays on the +4 that is because four other relays are soldered directly into the BCU

[Linked Image]

Photo is from my 2018 +4 and if you look closely bottom right it is marked +4 suggesting different BCUs for other models.

Tim
Plus 4 2018