spotter
noun
1
a spy hired by an employer to observe and report on employees' activities US, 1876.
2
a criminal who finds or identifies a likely victim for robbery UK, 1937.
3
a look out in a drug-selling operation US, 1990.
4
a trainspotter, a planespotter, a bus-spotter or a similar type of hobbyist. Spotter's Guides have been published by Mayflower Books of New York since 1979 US, 2003