I don't think it was a secret -- I think what happened was that they couldn't use her testimony UNTIL Amber Heard mentioned her the other week. I'm not 100% on the legal terms, but basically when Heard brought up Kate Moss it now made her a relevant witness in this case, and Depp's legal team is going to basically have her go against what Heard said. 

So Kate Moss is about to testify for the Depp/Heard trial. This is odd to me. So late in the trial. Like was this a secret and planned all along? What is she gonna say? Just when I thought I was out… they bring me back in again

Kate Moss 

Allison Piwowarski

When You Text "Nice," What Are You Really Saying?