How do you inject a feeling of strangeness into your game? Not outright horror, though it can descend to that. But the subtle feeling that something is not quite right, to get the players' attention without crossing over that line to the obvious? I find the use of the other four senses a good way of calling attention to something that gives a creeping suspicion that the players are walking into something on the wierd side. The faint sound of flapping in the wind like the wings of a wounded bird. The sudden rise of goosebumps as the wind chills a few degrees. The cloying smell of decaying flowers. The heat parching the throat, making the player swallow. The slimy feel of the bannister, leaving a bitter, tingling residue on the hand. Even better, use the senses in strange ways, twisting expectations. The taste of copper on the tongue, heavy in the air. There is also the negative use of senses. The most obvious used ones are the wildlife going silent, but an example could also be the sudden stillness of the air.
The use of metaphors or onomatopoeia (as long as they don't fall into overuse) can also signal that things are a bit off. The delivery of these concepts is as important as what is actually said- if you're trying to jar your players, then even how you say it should invoke that dissonance. "As you make your way through the forest- SNAP!" This communicates viscerally that the player's stealth was compromised. "As your movements disturb the night, the natural sounds die down, and you hear swift movement in the forest off of your left shoulder."
How do you communicate foreboding strangeness to your players?
I cannot overemphasize how effective a good piece of music with some sound effects can be. I'm a bit of an audiophile, so I really enjoy grabbing sounds and fiddling with them to produce various 'soundscapes' to evoke different moods for my players. The BBC has released a HUGE archive of sound effects that are free for non-commercial use here, and I've found literally hundreds of great sounds to use.
Many times, I like to tweak the sounds to sound slightly different or even completely alien (great for otherworldly or demonic creatures). My favorite free tool to do this is Audacity. After that, simply add some music - Tabletop Audio is explicitly suited for this, but there are many sources of free, public domain or otherwise open source music on the web, as well.
Interesting ideas ... I guess I'd just interject random observations such as "The door is rectangular. It has four sides."
In a video game or visual media, this wouldn't even register as strange. It's just normal. But if you're narrating to players that they notice "the door is rectangular" it's a red flag. It's like ... Okay ... umm ... that's normal. Why mention it?
@CitizenZero - Good point! I actually support Tabletop Audio because it's cool and I like to support cool stuff, but I haven't been able to integrate it into my games easily- mostly because they're virtual.
@Isaac Kuo - that's really sneaky! I have to remember that one- state the obvious in order to throw them off their game.