@scratch – all sorts of electronic information, radio, phone, most kinds of telemetry, just sniffing for conversations and casual chitchat waiting for a few unguarded words or sentences which can give clues to intent by the Russians. Conversations were picked up concerning the shooting down of the civilian airliner over Ukraine a few years back, which implicated Russian missile crews, along with photos taken of the missile launcher truck on its route out and back across the border, and research using online photographic resources matched them together.
The Russians aren’t quite as smart as they like to think they are; I reference the clowns who poisoned the Skripals, along with a police officer and the unfortunate woman who died in Salisbury. They were identified using CCTV and transit info, and despite using false ID were identified and their identities confirmed by a phone call to a relative!
Bellingcat is a good place to look.