the only thing I can spot on the go is that in vanilla Rome (guessing its the same for BI) every unit except siege-engines has the atribute;
-hide_forest, or
-hide_improved_forest

=======

**5 minutes and to custom-battles further down the line ....**

Mak you again are the finder of little oddities ..;)
see my lvl3 archer here, hadnt even noticed this before;


I can assure you they "can sap" as do their lvl2 brothers wich also show they can .... (similair units-stats, but +1attack, +2armour)

the stats;
Code:
type             roman archer auxillia
dictionary       roman_archer_auxillia      ; Archer Auxilia
category         infantry
class            missile
voice_type       Medium_1
soldier          roman_archer_auxillia, 50, 0, 0.4
officer          roman_early_standard
mount_effect     elephant +8
attributes       sea_faring, hide_improved_forest, hide_long_grass, can_sap
formation        0.8, 1.2, 1.2, 1.6, 3, square
stat_health      1, 0
stat_pri         8, 1, arrow, 170, 50, missile, archery, piercing, none, 25 ,1
stat_pri_attr    no
stat_sec         3, 1, no, 0, 0, melee, simple, piercing, knife, 25 ,1
stat_sec_attr    no
stat_pri_armour  8, 5, 0, metal
stat_sec_armour  0, 1, flesh
stat_heat        0
stat_ground      2, 0, 3, 1
stat_mental      10, disciplined, highly_trained
stat_charge_dist 15
stat_fire_delay  0
stat_food        60, 300
stat_cost        1, 260, 80, 40, 60, 260
ownership        roman


type             roman archer auxillia ii
dictionary       roman_archer_auxillia_ii      ; Archer Auxilia II
category         infantry
class            missile
voice_type       Medium_1
soldier          roman_archer_auxillia_ii, 50, 0, 0.5
officer          roman_standard
mount_effect     elephant +8
attributes       sea_faring, hide_improved_forest, hide_long_grass, can_sap, hardy
formation        0.8, 1.2, 1.2, 1.6, 3, square
stat_health      1, 0
stat_pri         9, 1, arrow, 180, 60, missile, archery, piercing, none, 25 ,1
stat_pri_attr    no
stat_sec         3, 1, no, 0, 0, melee, simple, piercing, knife, 25 ,1
stat_sec_attr    no
stat_pri_armour  10, 6, 0, metal
stat_sec_armour  0, 1, flesh
stat_heat        1
stat_ground      2, 0, 3, 1
stat_mental      12, disciplined, highly_trained
stat_charge_dist 15
stat_fire_delay  0
stat_food        60, 300
stat_cost        1, 320, 100, 50, 75, 320
ownership        roman

G