There are many circumstances that monsters may notice and call their targets out over such as stealthers being accused of hiding or someone hurting the taunting NPC.
Calling for help:
Many NPCs will also call for help if they know allies are nearby to assist them. No such things as Wolves assisting Bandits should happen however because racial relations are taken into account, meaning for example that Orcs will generally not call on Elves to help them and vice-versa. The same is also true for animals and monster or civilzed races etc..
In short: Don't count on being able to draw away a single opponent, if he's intelligent he'll simply call for help! Silencing him may offer a solution however.
When a monster says something there is a chance depending on the race that he will speak in his native tongue, meaning that the PC needs to know his language in order to understand what was said.
Sense of Smell and player character race:
It is also very important to be aware that some monsters have a very keen sense of smell, so keen in fact that they can determine your character's race and gender from his scent. Orcs, Trolls and Lizard kin are known to posses this ability. Most other monsters won't know these details about your character if you are wearing a face covering hood or helmet.
System curtesy of Rasael