וַיְחַפֵּ֕שׂ בַּגָּד֣וֹל הֵחֵ֔ל וּבַקָּטֹ֖ן כִּלָּ֑ה וַיִּמָּצֵא֙ הַגָּבִ֔יעַ בְּאַמְתַּ֖חַת בִּנְיָמִֽן׃
And he searched, beginning at the eldest, and leaving off at the youngest; and the goblet was found in Benjamin’s sack.
Rashi on Genesis
בגדול החל AT THE ELDEST HE BEGAN — so that they should not perceive that he knew where it was (Genesis Rabbah 92.8).
Radak on Genesis
ויחפש, בגדול החל, in order that they should not suspect him of having planted the goblet in Binyamin’s sack, he made a show of starting to search the oldest, etc. He knew their respective rank in seniority from the way Joseph had seated them at the meal.
Rabbeinu Bahya
בגדול החל, “he began by searching the sack of the oldest.” He did this in order that they should not realize the goblet had been a plant.