1 They are embarrassed
Some people, especially women, use the see no evil emoji when they are embarrassed, blushing or feeling shy. People use emojis to express their current state of feeling, and the see no evil monkey usually represents embarrassment.
2 They are avoiding the answer
Sometimes people use a emoji in order to avoid answering a specific question. The see no evil monkey might be used when a person is embarrassed, and at the same time wants to avoid answering a question.
3 They don't know what to say
Some people send emojis when they don't find anything to say. The see no evil monkey might be sent when the person gets stuck without finding a response to a question.
4 They think they did a mistake
A person can send that emoji when they believe that they did a mistake or something wrong that they are embarrassed of. It might be another way of saying 'Oops, I ruined something'. (See Why some people never use emojis?)
5 To make the situation lighter
Sometimes people use emojis in order to make a serious situation lighter. A person who made a mistake could use the see no evil monkey emoji to make the situation seem lighter.
6 To hint about what's coming
Sometimes people use the see no evil monkey emoji to hint that they did something they are embarrassed of. In such a case, the person uses this emoji to set the expectations of the person they are chatting with.
7 To tell someone you did something wrong
This emoji can also be used to hint to someone that they did something wrong in a funny and friendly way. In such a case, the emoji will be a response to a story that the person told.
8 I don't want to see that
The emoji could also mean that the person doesn't want to imagine something or see it because it might be too wrong or embarrassing.
9 To seem cute
Some people, specifically women, use emojis in order to make themselves seem cuter and more feminine. (See What does the laughing crying emoji mean?)