In one line
Bought or automated accounts that make an audience look bigger than it is.
A brand notices hundreds of followers with no photo, no posts and odd names, all arriving in the same week.
One odd sign is not proof. Look for several (sudden jumps, comments that do not fit the post, engagement far below the follower count) before you judge, and never accuse publicly without evidence.
Agree: ask for insights screenshots when something looks wrong.