I have a feeling that loved ones of addicts/alcoholics always have moments of panic when they either believe or know that their loved one has relapsed. Relapse is part of recovery for a majority of addicts (they S.L.I.P - SOBRIETY LOSES ITS PRIORITY).
How to avoid that panicky feeling? I guess just keep detaching with love. His recovery is owned by him and him alone. Your recovery, peace, sanity, and happiness belong to you. It's a lesson I'm still learning myself.
It sounds as though your husband had a legit reason for being out of contact for a bit. I wish you the best of luck as you both move forward in recovery!
Hugs, HG