I mean, not always. It's hard to be super secretive in a live situation, but I'd wager many musicians have successfully hidden their pirated plugins.

A lot of people have been caught anyways. Steve Aoki accidentally left a visibly pirated Sylenth VST in a promo vid, Porter Robinson and Skrillex both got caught with pirated plugins during track breakdowns, Kanye West posted a video with 30 tabs of The Pirate Bay open to download Logic Pro... the list goes on. It was extremely common in the early days of digital music production (and still is today, to an extent), but the backlash has pushed most legit production houses to legit licensed software.