As reported by Censor.NET, the officer was detained when transporting a load of ammunition to terrorists-controlled territory.
On the audio, two mercenaries with callsigns Malysh ('Baby') and Makeevskiy discuss Starkov's detainment. It follows from the conversation that this was not the first load convoyed by Russian officer Starkov to the terrorists.
One of the mercenaries also speaks of another terrorist who was captured by the Ukrainian side. The guy was supposed to deliver ammunition to the mercenaries.