an undercover agent or investigator, a police spy.
From French mouchard police spy, person who spies on someone from mouche fly + -ard.
“He’s convinced she’s put a mouchard on his tail.”
Itinerant (adj) (noun)
Travelling from place to place. / A person who travels from place to place.
Late 16th century (used to describe a judge travelling on a circuit): from late Latin itinerant- ‘travelling’, from the verb itinerari, from Latin iter, itiner- ‘journey, road’.