My coolant tank was empty and there was a plastic cane in the cockpit/dashboard, so I thought, it was water and I put some in the coolant tank. I am pretty sure it was less than 500ml.
Driver is using the vehicle, and it seems to working, engine/motor doesn't sound anything suspicious for 4 days.
Should I worry?
Does the motor has to be opened for clean up?
ADBLU is a mix of uric acid and water, and you don't want acid in your cooling system as it can cause corrosion. It's not a disaster, you just need to drain your coolant out of the system and refill with fresh coolant.
From your other question it seems like you have a cooling system problem and are losing a lot of your coolant every day. If this is the case I wouldn't worry about it as a mechanic is going to do this anyway to fix your other problem.
