Energy signature
Each Transformer has a unique identifying energy signature<sup id="cite_ref-0" class="reference"><a href="#cite_note-0" title="">[1]</a></sup>. That energy signature can usually be used to identify someone's faction. Certain substances or devices can mask a Transformer's energy signature from detection. The type of energy is unknown, possibly infrared.<sup id="cite_ref-1" class="reference"><a href="#cite_note-1" title="">[2]</a></sup>
Drones have energy signatures<sup class="noprint">[<span title="The material in the vicinity of this tag needs references to reliable sources." style="white-space: nowrap;">citation needed</span>]</sup>, so an energy signature is not the same thing as a resonant spark signature.
Fiction
Beast Wars
Blackarachnia developed a signature dampner which the Predacons used to hide themselves after faking their deaths. Victory
Beast Machines
All the Maximals' energy signatures were undetectable by the Vehicon drones when the Maximals were in beast mode. The signatures became detectable again whenever the Maximals were in robot mode.
Transformers Animated
Optimus was unable to detect Elita-1's energy signature when she was mutated into a half-organic being. This led him and Sentinel to believe that she was killed, causing a falling out between the pair. Along Came a Spider
Prime was able to detect Prowl's energy signature when the latter was consumed by a nano-machine-enhanced bug. He noted that it was weak, indicating a connection between the signature and the subject's health. Transform and Roll Out!
The Decepticon forces (all two of them) were equipped with dampeners created by Isaac Sumdac, which prevented the Autobots from detecting them outside of their makeshift base. Mission Accomplished
References
<ol class="references"><li id="cite_note-0"><a href="#cite_ref-0" title="">↑</a> To test the Predacon Autoguns, Megatron programmed a target box to emit the Maximals' energy signatures. Power Surge <li id="cite_note-1"><a href="#cite_ref-1" title="">↑</a> In the Beast Wars episode "Coming of the Fuzors", the Predacon security system identifies Dinobot's "Maximal infrared signature." It's unclear whether this is the same as an energy signature or the computer was simply programmed to recognize the infrared "fingerprint" of every known Maximal.</ol>
