Stalker works by monitoring your combatlog. When it hears someone close by it attempts to guess their level and class based on the spell that they cast. So if it hears someone cast a Rank 5 Siphon Life, it identifies them as a Level 63+ Warlock. Note the + after the 63. This indicates they may be higher, but we only know they are at least this level. Once you target or mouse over the player Stalker will be able to determine definitively their level and the + will be removed.