Can't Find in Yakronyms

Frequently seen in many postings is ":smt02" or ":smt04" etc. (or higher numbers) usually at the end of a sentence, and without quote marks. What's this mean?

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It's an emoji that didn't appear for one reason or another.
Not a yakronym. 

When you write a post or PM and select an emoji for a smiling face for example this will pop up as you are typing :  smt001

Then when you press the post button if you did it correctly the emoji appears on your post.

 :smt00 oops I mean