How do debt collectors skip trace?

Could you elaborate on the process of "skipping trace" that debt collectors often employ? What specific methods do they use to locate debtors who have gone missing or are evading their debts? Are there any legal constraints or ethical considerations that they must adhere to while conducting these activities? Additionally, how do advancements in technology, such as data analytics and online footprints, influence the efficiency and effectiveness of debt collection agencies in tracing debtors?