If a saboteur is one who sabotages, and a spy is one who does espionage, what do you call one who reconnoiters or performs reconnaissance?
Here is some research on the topic. Etymononline produces the following pairs, but not one for reconnaissance:
sabotage (v.) 1912, from sabotage (n). Related: Sabotaged; sabotaging.
saboteur (n.) 1912 (from 1909 as a French word in English), a borrowing of the French agent noun from sabotage (see sabotage (n.)).
espionage (n.) 1793, from French espionnage "spying," from Middle French espionner "to spy," from espion "a spy" (16c.), probably via Old Italian spione from a Germanic source akin to Old High German spehon "spy" (see spy (v.)). For initial e- see e-. Middle English had espiouress "female spy" (early 15c.).
spy (n.) mid-13c., "one who spies on another," from Old French espie "spy, look-out, scout" (Modern French épie), probably from a Germanic source related to spy (v.).
reconnaissance (n.) 1810, from French reconnaissance "act of surveying," literally "recognition," from Old French reconoissance "recognition, acknowledgement" (see recognizance).