Ever since the Giant War, the campers had become accustomed to the strange going-ons in Cabin 20.
It had started before the construction of the Cabin had even been completed. One of the campers who had been helping to build it had dropped one of the heavy stone blocks inscribed with strange words, and everyone within half a mile had turned into a tree. Luckily it had worn off after only a few hours, but Thalia, who was just passing by to say hi to Jason, was not impressed. After that, everyone was a lot more careful with the stones.
The first few times, it had come as a surprise, and nobody knew what had happened.
Not a week after the Hecate campers first moved in, there had been a loud pop and an explosion noise that nobody could find the source of. That night at dinner, the Hecate table was empty, but the plates of food seemed to eat themselves. The next day, Lou Ellen explained that her little half-brother had slipped and dropped powdered lebsoms into the marjorie juice that she had been preparing for a healing tonic Kayla had asked for, causing an invisible explosion that reacted with Hecate's blood to make all of the Cabin 20 campers invisible until she finished brewing the antidote that morning. Annabeth had to lend Lou her yankees cap so that Lou could see her hands whilst casting the enchantments - apparently it makes invisible wearers visible.
They didn't always find out what Hecate's children had done to cause... whatever happened in a particular instance.
A month later, there had only been a few minor incidents. So the campers were taken off guard by the suction of noise that made the entire area eerily silent until the following morning, when Lou Ellen was seen coming out of the Hecate cabin holding a bottle. Looking rather sheepish, she had uncorked it and all the hours of sound that they had missed came rushing out in a matter of seconds, like an audio call after the connection has cut out. Piper teased Leo that she didn't know it was possible for him to be so quiet. After they recovered from their shock, the campers waited eagerly for some form of explanation. But nothing came, and eventually they let it go.
But to be fair they might not always want to.
Long after the campers had accepted that weird things happened in Cabin 20, there came a day when there was a pink flash of light from its direction whilst all the campers were eating. Lou Ellen had sworn in some ancient tongue and hurried out of the pavilion, a couple of her siblings rushing after her. A couple of campers, determined not to be left in the dark this time, followed them to see what was going on. Not long after, there had been a second flash. After none of the campers returned, a few others dared to get close enough to see that all of the demigods who had left the pavilion had been turned into literal headless chickens and were running madly around the cabin like- ...well. When Percy told his sister Estelle about this, she giggled and told him that she didn't believe him.
There was a sort of unspoken rule of don't ask.
An incident that was unlike the others was at the campfire one night, the day before the summer boarders would be leaving to go back to school. All of the head councillors were called up by Chiron to give a speech, and as Lou Ellen made her way up, she tripped on a tree root. A glass vial fell out of her pocket and shattered on the ground, and a drop of its contents splashed into the fire. Everyone held their breath, but when nothing happened they listened intently to the councillors' speech instead. No sooner had the councillors finished speaking than the fire raged up and leapt off the pile of kindling. It grew a mouth and roared as it burnt up the ground between it and the campers, who were screaming and running. After several minutes of terror, Chiron managed to subdue it by smothering it with a fire blanket that he 'kept on hand in case of emergency'. When they had all settled down, one of the newer campers opened her mouth to ask. Jason caught her eye and shook his head.
After a while, the campers learned to ignore whatever happened and move on as soon as possible.
One time there had been a ripple of energy that came from the Hecate cabin. When the wave passed over a camper, they started speaking in a different language. The pulse bounced off the camp boundary and came back, prompting the campers to start up in a different language. This continued for at least 15 minutes until the wave started to lose strength and with each pass, the campers came closer and closer to English until the ripple finally disappeared. During the course of the incident, the campers had mainly stopped speaking and instead just started playing volleyball, or practising their archery. When they were all back to normal, classes just continued as usual. Leo confused everybody by continuing to speak Spanish for at least an hour after the incident ended.
Even the most unexpected things became expected.
The most recent incident could arguably be considered the weirdest. There had been a shattering sound, and Lou Ellen and her siblings were seen running out of their cabin, coughing as they tried to escape the smoke coming from inside. After nothing else happened, they continued with whatever they were doing. Later in the day, Lou needed to gather some plants from outside the boundary. Will and Cecil went with her to keep her company. They were gone for about an hour and a half, but when they returned, they couldn't get through the barrier. Will, in an attempt to attract some attention, shot a flaming arrow over the barrier. Or at least, he tried to. It ended up going so wide that none of the campers even noticed it. With no way to get into the barrier, they began to worry about monsters finding them. But several hours later, there were still no monsters in sight. It was at that point that Lou Ellen told her companions that she'd been experimenting with the Mist lately, and she thought she might have accidentally made herself mortal - and it was contagious. That one took a while to sort out (they may have ended up moving to the city and living as mortals for about a month until the moon was in the right position), but eventually it was over with, and the campers just accepted that and moved on.
That was why, when IT happened, none of the campers spared it a second thought...