sidler

/ˈsaɪdlər/
noun
  1. A person who moves in a quiet, cautious, or sneaky way, especially when approaching someone.
    • The security guard watched the sidler as he crept along the wall.
    • The detective noticed a sidler near the crime scene and decided to question him.
    • In the crowded market, a sidler tried to get close to tourists without being seen.