Sorry for the late reply. Found it about 10 days ago.
It was the V5 water drain valve! Apparently during the leakage test this valve (among other things i.e. bowl, V4) should be 100% closed and of course not leaking. If it leaks then the pressure decreases and the system gives an "Oil leaking from bowl" alarm.
How i found it?
Well i simply opened the cap of the discharge sludge pipe during Ti66 (leakage test) timer and while PT4 pressure had a value of about 2.5 bar, i noticed water and then oil coming out from the flexible outlet hose of the V5. I guessed "this shouldn't happen, V5 (Normally Closed) valve opens only upon discharge (Ti73) and on some occasions when water is detected in the outlet and definetly not during Ti66"
So we overhauled the faulty V5 valve where we found a small piece of some kind of o ring stuck into the piston which obviously was the reason for the abnormal and random behaviour of the valve. We then tested - pressurized (@ 8 bar) the valve with success, placed it back and everything is working fine 10+ days 24/7 without the slightest alarm! Note that in our installation the flexible outlet hose of V5 valve is connected to the discharge sludge pipe which in turn is connected to a sludge tank.
Lesson: "Oil Leaking from Bowl" alarm especially upon discharge process could come from a faulty V5 valve as well. You won't see this written in any Alfa laval manual and you might not hear it from any alfa laval expert either.
Precaution: When removing the cap from the discharge pipe during discharge procedure, make sure that you are in a good distance (2 meters at least) because you can get easily injured from the ejected sludge products upon bowl opening! I had my own unpleasant experience so beware!
@Revolver: Ti65 = Test of water content in oil outlet. V4 was checked no leakages from this valve. Sensors all cleaned no issues.
@Atlantic: Thanks for the suggestion. Although bowl speed was fine we checked the belt and pads and found nothing unusual. Otherwise i guess a "low bowl speed" alarm would have occured at some point.
@SanMarino: Yes one of our earlier actions was to replace SV10 flow valve and also SV15 and 16 valves as well.
@sts07: I "feel" you my man. We have dealt with the same issue many times. That spring in the solenoid valve and the piston in the actual valve could affect opening/closing thus the water flow of SV16 (closing water) and cause problems. Buy some water valve kits and use them accordingly.
Sometimes these valves leak as well causing additional problems. There is a very simple, quick and effective way to check this instead of checking manually the valves 1 by 1:
Place a ball valve and then a 0-6 bar manometer just before water inlet to check SV water valves for leakages. Simply close the ball valve for a few seconds during normal operation (and not during discharge) and observe manometer. If pressure is stable then SV valves are fine. If pressure drops rapidly then some valve is leaking. In our situation SV10 has presented the most leakages. Have your shift guys do this simple check periodically i.e. 3-4 times per shift.