Shadowing

Shadowing differs from sneaking because the intent is to follow the target unnoticed in a crowd rather than avoiding being seen entirely. This requires an opposed roll of Wits + Stealth for the shadower against the target’s Perception + Alertness. Difficulty is normally 6 for both parties but this can be adjusted based on the size of crowds and the relative familiarity of the terrain for both parties.
Generally bigger crowds are easier to hide in than smaller crowds. If the shadower is a different race or otherwise looks substantially different from the crowd she is trying to blend in with, the target's Perception + Alertness difficulty goes down.
If the target achieves more successes than the shadower the tail is spotted, otherwise the shadower remains unnoticed. Groups of pursuers can take turns shadowing a targeting “handing off” the target periodically to prevent the target from noticing one person repeatedly but this takes special training on the shadowing group’s part.