Could it be UTL? That would be Unable To Locate. Don't know ETL either.
Enroute To Location
Why not just say en route, less syllables. Obviously it's a location.
As Carlt stated it is Unable to Locate. In the context you stated "FSP is ETL, I'm 97 98"
Freeway Service Patrol was Unable to Locate the vehicle and the CHP unit was 10-97 - 10-98 and cleared.