itch.io

Thanks! There are textual hints after you die, but I should have probably included more detailed instructions into the game.  I've put it into a wip thread in jam's community section: https://itch.io/t/5320299/shut-wip (one of the longer posts below)

In short: 

There's about 50% chance that a room contains a hidden Remnant. A Remnant will attack and insta-kill you if it's in the room and you've shut all of the room's windows. So you need to dispose of the Remnant before closing off the room.

Only the parts of a Remnant that are hit by direct sunlight are visible. 

To spot a Remnant, you need to shift the sunlight in the room by "waiting" (hold RMB) in order to "scan" the room with light. Sorta like a natural searchlight. Look for parts of Remnant's black silhouette that are lit by sunlight. If you see such a part, approach and click on it to dissolve the Remnant. The reticle will turn magenta when you can do that. Sometimes it can only be a tiny part but sometimes the whole silhouette will be visible, depending on the light pattern that shines on it. After you dissolve the Remnant, the room is safe to be shut. 

The more open windows in a room, the easier will be to locate a Remnant or conclude there isn't one, as you have more light to work with

The tricky part though is that light also damages you. The more light (total window area) in a room the greater the ambient exposure that drains you. On top of that, direct sunlight (when you can see the sun through a window) will drain you very quickly. This is indicated by the sun icon above the exposure bar. So try to avoid that as much as possible.

So you need to work with light to spot a Remnant but also minimize your exposure to it.

Last but not least: You can regenerate. Close yourself into a room with all windows and all doors shut and hold RMB to gradually restore soul. This will use your time resource though.