[RogerBW]

I think the "be seen on a failure by 5+" is meant to reflect the situation one sees in film: A is following B, A loses track and hurries on to try to catch up, B spots A bursting out of the crowd/alley/whatever.

I think it's a shame the two aspects (keep track of someone, don't be noticed) are conjoined. I think I might try a three-way contest of skills: Shadowing vs Shadowing or Stealth to keep track of the target, Shadowing vs Vision to avoid being seen, using the same Shadowing roll for both comparisons. As things stand, you don't get the situation where B spots A but A doesn't know that it's happened, so B can lead A into a trap.

Similarly I'd be inclined to use an appropriate vehicle skill as a Complementary Skill for tailing by car, motorcycle, etc.