Yes I'm sure, because I found the plug under the car afterwards. At first I didn't want to plug it too with the same thing in my mind, however, at volvo speed web site, and haynes manual it was said that I should plug it. Maybe I've understand it wrong because of my broken english but I've juzt checked Volvo speed and it clearly says "Remove the battery and battery shelf. Remove the return hose on the control system (valve body). Install a plastic plug in
the connection on the cover (valve body) and a transparent plastic hose on the oil return hose connection. Position a
container for the oil with markings for the volume under the hose. Temporarily reinstall the battery shelf and the battery." Anyway, I'll go home and check the fluid level again if it's same as yesterday. Tomorrow I'll post another update on it.
Thanks,
Ihsan