Baston,
I never dimiss DR. Heading, Airspeed, Timing and Stopwatch at all times.
Even with the best GPS / IN available. I would wager a bet that the crew were doing just that.
I pride myself to think that I could considerably better that 4 nm circle of uncertainty by a considerable margin.
Would I spot an error of 1.5nms after 15 minutes over water with all the variables including W/V. Perhaps.
But remember I was only giving an example.
What if the "coarse turn" was planned at 1.5 nm and the TANS was in error by 0.9nm.
Still feel they would spot it.
Regards