When the LORD called young Samuel at night, who did Samuel think was calling him?

Answer: Eli

Samuel ran to Eli three times saying, "Here I am," until Eli realized that it was the LORD who was calling the boy.

“He ran to Eli and said, “Here I am, for you have called me.” “I did not call,” Eli replied. “Go back and lie down.” So he went and lay down.”— 1 Samuel 3:5 (BSB)