Probably anything she ever said to any Team Flare member after she left the organization.

There's no doubt that, even if Briar tried to go unnoticed by the Interpol, sooner or later she would end up meeting someone who worked in Team Flare with her. Someone that recognized her and tried to speak to her, but Briar, who was incredibly afraid of being discovered by some agent, ended up acting cruel and heartless towards any former member of Team Flare.

"Why are you speaking to me? I never cared about you. Leave me alone."
"We are not friends. We were NEVER friends."
"You were all part of a business for Lysandre, and for me as well. So don't think just because we worked together in the past you can come and talk to me like we're buddies. Get lost."

And the truth is that she cared for Team Flare. But maybe the whole experienced caused her to become colder, and knowing that the Interpol might get her at any moment, she prefers to look out for herself before having to look out for another Team Flare member. So sometimes she has to hurt people in order to achieve it.