Indeed. Visible, as opposite of invisible, can't be read at client-side.
On the other hand, we have OnScreen property at step 164. This is used to track only valid creatures (alive and on screen) that you can attack.
Code:
Tibia 10.90
BL_Start = Tibia.exe + 72DE20
Step = 220 (0xDC)
ID = 0
Name = 4 (0x4)
Z = 36 (0x24)
Y = 40 (0x28)
X = 44 (0x2C)
Facing_Direction = 56 (0x38)
IsWalking = 80 (0x50)
Walking_Direction = 84 (0x54)
Outfit_ID = 96 (0x60)
Outfit_Head = 100 (0x64)
Outfit_Body = 104 (0x68)
Outfit_Legs = 108 (0x6C)
Outfit_Feet = 112 (0x70)
Addon = 116 (0x74)
Light_Radius = 124 (0x7C)
Light_Color = 128 (0x80)
HP_Percent = 140 (0x8C)
OnScreen = 164 (0xA4)