If the foreign service officer is returning from leave/R&R to where he or she is assigned, they would use their diplomatic passport, which has their residency permit, to enter the country of assignment. Also, R&R travel is theoretically official travel, so they would be using their diplomatic passport. Either way, there is no requirement to be dressed in a suit, plus at some tropical or out of the way locations, they might not even wear a suit on a daily basis, it's not all Europe for our foreign service officers.