Researchers at Ben Gurion University in Israel have demonstrated an attack that allows malware to communitcate between air gapped computers in close proximity by using the machine's onboard thermal sensors. The demonstration required somewhat controlled conditions and only achieved a transmission rate of eight bits per hour. The slow rate of transmission could however over an extended period of time be sufficient to leak valuable information including cryptographic keys.
Thermal communication has been used before and the Ben Furion demonstration merely represents a new instance of a particular type of this. This communications channel presents a low risk in the wild as mitigating measures are trivial, but it is an avenue for communication that should be considered when establishing a secure computing environment. There are many factors to be considered in airgapping which merit greater time and effort. Radio Frequency, acoustic emissions, and electric supply noise particularly merit far greater time spent on mitigation.