THANK YOU FOR COVERING FOR ME DURING MY PERIOD OF ABSENCE
I just wanted you to know how much I appreciate the extra effort you made to cover for me during my period of absence.
I know how hard you already work, and I am well aware of the additional demands which my workload placed on you.
I have acknowledged the extraordinary efforts you made, and they will not be forgotten.
I hope to be in a position to reciprocate whenever you need me. Thanks again.