Sforno on Exodus
יצאו מן העם, from their camp to the area where during the week the manna had been falling. These people thought that they would find some manna, possibly outside the usual parameter. This was a demonstration of lack of faith in the word of G’d.
Or HaChaim on Exodus
ולא מצאו, and they did not find any. The Torah means that these people would have collected it if they had found some.
Mekhilta d'Rabbi Yishmael
(Exodus 16:27) "And it was on the seventh day that there went of the people to gather, and they did not find": These were the faithless of Israel.