In mystery thriller film Searching (2018), Kim contacted his missing daughter’s friends so he can know where they were when his daughter went missing. He wrote their locations on a google sheets, but for one of them he wrote “confirmed”. He made said list after calling each one. So why did he mark one of their locations as “confirmed”?
Kim wrote (Confirmed) because Derek was his likeliest suspect at the time and he needed to vet his alibi.
Unlike previous people he'd contacted, Derek had been antagonistic towards Kim and had catcalled Margot on her social media profiles before she was taken. Kim suspected that Derek was lying to him and thus had to perform additional validation of his alibi. He didn't have such suspicions about Margot's other peers and didn't need to spend valuable time confirming their backgrounds too.
It's also a small joke that such a tough acting teen would attend a "girly" Justin Bieber concert.