I must have dropped off to sleep.

我一定是睡着了。

《柯林斯英汉双解大词典》

Oh, I must have dropped it.

哦我一定是把它弄丢了。

youdao

Must have dropped it this afternoon.

一定是今天下午丢的。

youdao

'I believe I must have dropped asleep, ' it said, stretching itself.

NPR: Excerpt: 'Five Children and It'

Days earlier, Ms Battle had filed a police report saying she had picked the winning numbers but believed she must have lost the ticket when she dropped her handbag in the snow outside a convenience shop.

BBC: NEWS | Americas | Women argue over 'lost' jackpot

The 10-year Treasury note must have thought it was already in Tokyo: the yield dropped to an unbelievable 1.45%.

FORBES: Student Loan Bubble Sets Up To Be Subprime Disaster Part Deux