Exploratores were the scouts and reconnaissance element of the Roman army and were tasked to keep watch on enemy movements in the field. Exploratores comes from “explorare”, to scout.
The exploratore unit originated from the cavalry; it already had the great advantage of mobility. The typical soldier assigned to this unit would generally be native to the area due to his knowledge of the terrain. The term Numerus refers to an army unit of 200 to 400 men, normally auxiliary. The number varied depending upon the duties of the unit. To this day, we still have little evidence regarding the size and organization of an exploratore unit; they were thought to be relatively small in size and because there was no official ranking for them, by default, they were under a centurion (Ezov, 79).
The evidence for these units in Britain comes from two separate sources.
Epigraphic evidence dateable to the early-2nd century recovered from two garrison forts in Northumberland.
Numerus Exploratorum Habitanco – The Company of Scouts from Habitancum
[...] SARMATI[...  ...]INI PII PRO[...   ]ADRIANI A[...  ...  ...]I PARTICHI ET [...  ...]
[  ... ] ANTON[...]ICO MAXIM[  ...  ...] TRIB POTESTA[...]
[...]RIE PROCONSVLI PRO [...  ...]OTIONE COM[...   ]T IV[...  ...  ...]TRI AVGVST[  ...]
[...]M SENATVS HAC PATRIE PRO [...]ETATE HAC D[...]ONE [...&
RIB1243 - Fragmentary building inscription
[...]R COH [...]
[...] SOL RE[...]
RIB1244 - Inscription
[...] CO[  ...]
[...] ET N[...]
[...] ET EX[...]
Habitanco is known to be the fort at Risingham from the following inscription:
RIB1225 - Altar dedicated to Mogons Cad(...) and to the Divinity of the Emperor
MOGONITO ❦ CAD ❦
ET N D N AVG
M G SECVNDINVS
[...]F COS HABITA
NCI PRIMA STAT
PRO SE ET SVIS POSV[...]
Numerus Exploratorum Bremenio – The Company of Scouts from Bremenium
Bremenium is known to be the Fort at High Rochester.
Documentary evidence from the late-4th century Notitia Dignitatum which records one unit under the command of the Count of the Saxon Shore in southern England and another “at the disposal of the Right Honourable Duke of the Britains” in County Durham; the former two units separated by a distance of only 10 miles, the latter two by almost the entire length of the province. There is no evidence to link either of the 2nd-century units to either of the 4th-century units.
RIB1262 - Altar dedicated to Genius of our Lord and of the standards of the First Cohort of Vardulli and of the Unit of Scouts of Bremenium
COH I VARDVL[...]
ET N EXPLORA
TOR BREM GOR
ANVS LEG AVG PR PR