The so-called side-channel attacks – in which attackers steal information from a machine they don’t have direct access to – are getting more popular these days. This is another innovative example of a side-channel attack. The target laptop was not connected to any network (either through LAN or Wifi) and yet the attackers managed to obtain the encryption keys from it.
“The attacks are completely non-intrusive: we did not modify the targets or open their chassis.”
Source: How White Hat Hackers Stole Crypto Keys from an Offline Laptop in Another Room | Motherboard