Even Zero and Lucas did not have any watch with them but they can still predict it would look like it was almost 2:00 a.m. Naturally, both of them did not sleep or anything like that. First of all, that would be the dangerous thing to do inside the Dungeon.
On the other hand, they already sense the malicious intention of the group of guardians. So they can already predict that those people will attack them. They are also ready to counterattack those people.
Around 2:00 a.m. those 10 Guardians finally begin to approach the direction of Zero and Lucas. Well naturally they can take action more quickly but all of them decided to wait for the right moment to attack those two people. Even though there is a chance that those people have already been abandoned by their team, both of them are still present there.