Wiesel learns much later that he and his father missed an opportunity by not staying in the hospital during the camp’s evacuation. Explain.

This happens in the Buna camp.  Essentially, Eliezer's foot worsens during the cold weather and must have surgery and is hospitalized.  The particular time is important.  In 1944, the war was entering its final stages in the European Theatre. As the Nazis were in retreat, many SS officers were under orders to eliminate the remaining inhabitants of the camps.   When Eliezer, in the hospital, hears of the Russian troops coming to liberate the camp, he believes that they Nazis would rather kill off those who remained that allow the Russian troops to liberate them.  It is with this mindset that Eliezer and his father elect to leave, believing that the sick and those who were bed ridden would be one of the first to be executed by the  Nazis with the impending Russian liberation of the camp.  The realization that hits later on is that there was not any mass execution, and had they stayed in Buna, they would have been freed.

This reflects the desperate calculation that many during the Holocaust were forced to undertake in a situation that embodied chaos.  Wiesel refers to this in the work to show the level of rationality present, and how individuals who have knowledge and understanding, forming seemingly logical and plausible conclusions, could not predict the Nazi actions.  If we extrapolate this, the whole notion of "selection" is one that is rooted in supposed rationality, but is quite random.  The belief was that the Nazis used some level of "criteria" to determine who would live and who would die.  However, if one was placed as a prisoner of the camp, there could be no way one would be able to accurately predict if they were to live or die.  This becomes the terror of the Nazi philosophy:  It is supposedly steeped in rationality, but is highly surreal and random without any measure of deduction evident.  In this same manner, of rationality masquerading as irrationality and vice versa, Eliezer and his father make a reasonable and calculated decision that does not materialize.