From Halopedia, the Halo wiki
Tykos 30 is an organic tracking device. It has a similar appearance to the yellow liquid "blood plasma." One Marine described to a man injected with the substance that they "can track ya to Mars." The UNSC believed that the Covenant were not aware of its use and were consequently unable to track the substance. In reality, they could, but they misunderstood its true function.
List of appearances