If someone is absent from a meeting, ceremony, or place, they are not there.
Gary O'Neil has been absent from training because of a stomach virus.
Their children are frequently absent from school.
You use from after absent in sentences like these. Don't use ‘at’.
If it is clear what meeting, ceremony, or place you are talking about, you can simply say that someone is absent.
The Mongolian delegate to the assembly was absent.
‘not at’ and ‘not there’
Absent is a fairly formal word. In conversation and in less formal writing, you say that someone is not at a meeting, ceremony, or place, or that they are not there.
She wasn't at Molly's wedding.
I looked in the kitchen but Magda wasn't there.