Is a US Coast Guard mariner employee required to have in his possession a card verifying that he/she is enrolled into a drug testing consortium?

There is no regulatory requirement for an employee to carry a form of proof that they are a member of a consortium.

There is no regulatory requirement for an employee to be a member of a consortium. It is the responsibility of the marine employer to provide records as acceptable proof that all safety-sensitive employees are subject to a valid Random Testing program.

In those cases where the individual is an owner-operator, then the USCG Inspector or Investigator may require the individual to produce records proving compliance with 46 CFR Part 16.203 and 16.230