Recently, I was watching
a video by Lindybiege about lighting fires - and more importantly, about how they generally didn't light fires: it's apparently very difficult in anything other than ideal circumstances (rather more-so than RPGs have lead me to believe), so what they did was never let their fires go completely out. Even if they did, you mostly had neighbors who you could bum a light from, on the assumption that if they ever needed it, you'd do the same for them.
I'm pretty sure GURPS has rules somewhere for 'carrying glowing embers with you all day so you can light a fire when you arrive at the next camp,' but that's not what I thought of for this thread. Suppose that one night, every fire in the village goes out all at once, and when they search for embers to relight their fires with, they find that all the hearths are stone cold. It's not winter, so this is not a complete disaster, but they still need to get someone to the next village in a bloody great hurry (if there's a castle nearer, of course, that's the first place they'd go, but let's say that the lord of the manor, if any, hasn't got any fires or embers, either). They also go and look to see if anyone is missing, or if any strangers are about, and send runners to outlying farms and such to see if they've got any fire.
Regardless, this is a bigger problem for them than it would seem to us, even if they do have someone who still knows how to get a fire lit without an ember. They know fire, and they know that hearths all going stone cold like that
does not happen. They are very scared, and scared people are dangerous, especially if the PCs just got in last night (strangers in the village are likely to be blamed for things suddenly going wrong after they arrive, especially if they're
very strange).
Let's say that they get the fires lit, things are normal for a few days, and then it happens
again. The local priest (or priests, and/or priestesses) is likely thinking that some deviltry is afoot, everyone is looking for someone to blame, and the local lord wants answers. If it keeps happening, and especially if folk start dying or disappearing, you've got some serious problems.
You could get a pretty good ancient, medieval, or even renaissance horror game starting from here, but what are some other examples of spooky things that don't directly cause damage, but would be more threatening and frightening at lower tech levels - especially things that would actually be more inconvenient and/or scary than we would expect?
EDIT: Important thing I forgot to say about the scenario, the hearths stay stone cold unless actual fire (or a glowing ember, or a red hot piece of iron) is applied to them. Not red hot tin or something, has to be iron.