They had appeared! The Death Eaters patrolling Hogwarts Castle late at night felt the searing heat from the Magic Coin in their hands and realized that Dumbledore's Army had indeed emerged from the Room of Requirement, intending to head to the castle dungeons under cover of night to rescue people.
However, just as all the Death Eaters received the alert and their spirits rose, no second alert came. That meant they had either failed to catch anyone, or the enemy had already headed toward the dungeons.
Snape had improved this method of relaying information based on the way Malfoy had previously described Dumbledore's Army using their coins.
As for using Communication Bookmarks to relay messages like the Defence Association did, the Death Eaters wanted to as well. But those things clearly could not be made in a short time. Most Death Eaters were better at Dark Magic than Transfiguration Charms, and even if they threatened Professor McGonagall into making Communication Bookmarks for them, they might not dare use them.
"Looks like the Death Eaters have discovered us."
Halfway to the castle dungeons, Arthur noticed the red light beside his watch. He stopped and turned to look at his sons.
"The Death Eaters probably have some means of communication too." Fred and George exchanged glances, unsurprised. "From the beginning, we guessed this was a trap. Since it's a trap, they must have a fast way to communicate, so they can surround us from every side."
Arthur and Bill were both rather astonished. The Defence Association members remained remarkably calm, as though being discovered by the enemy had been expected all along. They were very curious what this group normally did to seem even more experienced than they were.
Perhaps noticing their surprised looks, George chuckled and lowered his voice. "The worst-case