I always thought FISO were supposed to keep a lookout for traffic within the ATZ,
2 Responsibility of FISO
2.1 The FISO has the following specific responsibilities:
a) issuing information to aircraft flying in the aerodrome traffic zone to assist the
pilots in preventing collisions;
b) issuing instructions and information to aircraft on the manoeuvring area to assist
pilots in preventing collisions between aircraft and vehicles and obstructions on
the manoeuvring area or between aircraft moving on the apron.
c) issuing instructions to vehicles and persons on the manoeuvring area;
d) alerting the safety services;
e) initiating overdue action;
f) informing aircraft of any items of essential aerodrome information.
Even if a FISO was to keep a lookout for traffic CAP 410 states:
However, with the exception of issuing instructions to aircraft on the ground under the
conditions described above, FISOs must not issue instructions of their own volition
or exercise any form of control over aircraft. It should be noted that neither the
aerodrome authority nor the FISO can prohibit an aircraft from entering the aerodrome
traffic zone.