Put in a sign is kinda tricky. We're dealing with unscientific danger here, it might even increase people curiosity.
No, you're missing the point: in order for anyone to have gotten that far to the place where the sacrificed sealing monk is there at all, they must
already have some mojo. That's not "idle curiosity" that's breaking into a place, breaking through a false wall, breaking through any magic repellent, and getting to the final point. In the military analogy, it'd be like someone who had already bypassed wire fences, surveillance systems, armed guards, and fortifications to make their way all the way inside to the point where weapons are kept. Like, they didn't wander by walking their dog. If they've gotten that far at all then it's now one step away from danger and there are two possibilities:
1) It's someone who knows what they're doing and intends to break that seal on purpose. A sign causes no harm here, because they know what they're about already.
2) It's someone with mystic sense who got lured in somehow, like our humble party of adventurers here. They know magic ghost juju, but aren't actually malicious. They may (and again, this is what actually happened) think they're doing the right thing. This is where full disclosure, or better have actual serious defenses, is important.
The context here is a tactical WMD level demon god that an organization has been keeping sealed for centuries. It definitely feels like they were taking it curiously lightly. I guess you could argue they just got sloppy but man I dunno. Like, why even allow a road to go by there, why not have it be the center of some sanctuary? It was all rural way back in the day, they could have worked something out centuries earlier when it was cheap/free. Hence why it'd be interesting if it turns out to be subtle corruption, because in turn that might point to other aspects in human society, various active gods or ancient horrors that are taking advantage of twisting bureaucracy or whatever without anyone realizing it.