Sam wanted to go up to the man but suddenly he_____that there was none in the house.

noticed 注意到

选noticed.
原因：NOTICE vt.
注意,注意到[W][O3][+v-ing][+wh-][+(that)]
She didn't notice that I had entered the room.
她没有看到我已经走进屋里。

found out，发现的意思

noticed