Senior Agent is a rank used by several law enforcement agencies within the United States.
In particular, in the Louisiana Department of Wildlife & Fisheries - Enforcement Division, it is a rank below Sergeant, and above Agent. This rank has no rank insignia. Agents who complete two years of satisfactory or exceptional service are promoted to the rank of Senior Agent.
Usage in other agencies or countries may vary.
References
Senior agent Wikipedia(Text) CC BY-SA