OK. Instead of if if if, could could could, should should should , etc, etc put yourself in Stena OOW shoes . Lets assume He was 3rd mate rookie with little practical experience but was a top gun at academy/school so He knows theory like the back of his hand.
Scenario: milk shake fog, your vessel filled with 35 k mt of jet gasoline( a ticking bomb) is anchored with 6 shackles- manual brake engaged , currents , E/R notice is 1 hr, anchorage= customary place where ships anchor off port waiting for berth, in UHF reach of VTS, Pilot stn, Harbour office, Pilot stn. UHF at ch 16 and DSC scan
From NP publication /pilot books/guide to port entry and info from agent You know the area is frequently crossed by coastal traffic .
You have S & X Radar arpa fitted on, Ecdis on , AIS on , your crew is busy doing their thing on deck and in the E/R, your fog signals are …lets skip that for now.
X radar is on 3 miles range , S radar is on 6 miles but you switch from time to time to 12 Nm range as you are a curious dude.
Suppose You are doing nothing else but your eyes are glued with heroic dedication to radar screens, ecdis and from time to time glancing at your nautocon screen and your hand held radio is on ships working channel to maintain contact with bosun ,a/b watchman if necessary. With your naked eye you see as far as forecastle.
You have detected a target with v=16 kts at 12 miles showing
CPA abt 0 Nm fluctuating . You do your math quickly in your head and it tells you she covers 1.6 Nm in every 6 minutes.
What would You do ? 
- at 8 Nm CPA =0
- at 6 Nm CPA =0
- at 4 Nm CPA =0
- at 2 Nm CPA =0
- at 1 Nm CPA=0
- at 0.5 Nm CPA=0
- at kaboom
When would You call the master??