I heard on the radio, The Dana Show IIRC, that since his incarceration in Mexico; he’s received orders to report for duty. Now supposedly the Corps know where he is, so why the orders?
He’s in the Individual Ready Reserve because he’s completed his active duty service, being in the IRR he doesn’t go to monthly drills or 2 weeks annual training.
The letter to report could be for checking his physical fitness, for administrative training or I imagine other things. It could be for only a day and doesn’t mean he’s being called back to active duty.
The Marine Corps is fully aware what has happened to him, hopefully that letter is not a legal requirement to start a court-martial or discharge process which I don’t personally believe it is.