Most people thought the Dark Lord wasn’t dead - he could return.
Dumbledore wasn’t the only one who thought the Dark Lord wasn’t dead - when he killed the Potters, most wizards thought he’d lost his powers but not actually died.
“Professor McGonagall’s voice trembled as she went on. ‘That’s not all. They’re saying he tried to kill the Potters’ son, Harry. But – he couldn’t. He couldn’t kill that little boy. No one knows why, or how, but they’re saying that when he couldn’t kill Harry Potter, Voldemort’s power somehow broke – and that’s why he’s gone.”
- Harry Potter and the Philosopher's Stone, Chapter 1 (The Boy who Lived)
Knowing that, Dumbledore likely thought the Dark Lord would return was inevitable - he’d lost his powers, not his life, so he’d be trying to get them back, and Dumbledore didn’t underestimate him. Whether Dumbledore had realized that the Dark Lord might have survived it because of Horcruxes or not, he knew the Dark Lord wasn’t dead so it was possible, even likely, that he would return.
Dumbledore might’ve also inferred he’d had Horcruxes by then.
Though most wizards suspected that the Dark Lord wasn’t dead, it’s likely that Dumbledore already had his suspicions about his methods used to enable him to survive it by then.
“I have been hoping for this piece of evidence for a very long time,’ said Dumbledore at last. ‘It confirms the theory on which I have been working, it tells me that I am right, and also how very far there is still to go …”
- Harry Potter and the Half-Blood Prince, Chapter 23 (Horcruxes)
He’d certainly suspected what Harry’s scar meant, so it’s very likely he’d suspected the Dark Lord had created Horcruxes then as well - that would explain why the Dark Lord’s soul was fragile enough that a piece of it could break off without him knowing and embed itself in Harry.
“I guessed, fifteen years ago,’ said Dumbledore, ‘when I saw the scar on your forehead, what it might mean. I guessed that it might be the sign of a connection forged between you and Voldemort.”
- Harry Potter and the Order of the Phoenix, Chapter 37 (The Lost Prophecy)
This would make Dumbledore even more sure the Dark Lord would return than anyone else - having Horcruxes would mean the Dark Lord couldn’t die unless they were all destroyed, so there was no hoping he’d simply die before he finds any way to regain his powers.