Originally referring to a CIA spy but has come to refer to anyone in the Law Enforcement business. Anyone who shadows someone is a spook.
1. Police officers, security guards, airport/border customs officers, private investigators, insurance fraud investigators, debt collectors, repo men, neighbourhood patrollers, etc.
2. snitches and informers.
“Spooky the spook is a paid informer.”
“That same spook is following me around again.”