check your rear tire pressure. It could be steering from the rear with a low tire. Also if the car is new to you, check to see if the rear subframe/radius arms are not bent.  You can measure corner to corner on the subframe to see if it's square. Radius arms will require measuring from the body and comparison of both sides.

I had a set of tires (Goodyear Double Eagles) that were extremely sensitive to tire pressure. A 1 psi difference between the 2 front tires made the car pull hard in the direction of the low pressure tire. I once had 3 psi difference after leaving the car sit for a couple of months and the steering wheel required a 90° correction to keep it going straight. The Yokohamas I'm running now aren't nearly as sensitive to tire pressure.

My primitive method for checking the toe angle front and rear is I stretch a string around all four wheels at a height at the middle of the wheel. I then place four 1" blocks of wood under the string, one on each of the four tires, at the most outward point towards the front on the two front tires and towards the back on the rear two tires. I then can check the gap on the other parts of the tires with another 1" block of wood. Randy

As you said it is pulling strongly even on flat ground and alignment appears good, I would suspect rolling resistance in one of the right wheels.

The first thing to check is tire pressure and condition - any signs of damage to the tires?
Check that brakes are not dragging, and that when applied and released, they really do release.
Check the wheel bearings are in good condition, are lubricated and have the correct pre-load.
Going deeper, check that the front axles turn equally freely and that there is not excessive resistance in the flexible joints (CV joints and pot joints) or in the differential.

I have checked and rechecked my alignment settings on my 1992 mini and they all look reasonable and what I had set them to:
Camber ~-1 degree all around
Caster ~+3.5 degree
Toe: ~1.5mm out front, ~1.5mm in rear
both left and right side are reasonable close to each other for all readings. 

Somehow my car is still pulling to the right fairly strongly even on flat ground.  I measure the toe using toe plates, which tells me my toe in or out, but I'm not sure how I ensure that both wheels are pointing straight and not cocked to one side and still giving me good toe readings.  I would imagine both wheels should be point straight forward, but if both are pointing slightly to the right, I could still get 1.5mm toe which would seem correct.  Does this matter?