Hiding Tiles

This is technically a core tutorial, but I'm including it in MATT because, well, that's probably why you're here. You have several options to hide tiles even from the GM view.

Toggle Visibility

Right-click the tile, click the little cloaked guy. Players won't see it, but the GM still will (though at 50% opacity). This is intended core Foundry behavior, as tiles are not something Monk invented, despite how many people seem to think so. Foundry's only intended use for them is to display images on the scene, which is why there they don't include a method to hide the bounding box if no image is set.

Transparent Image

MATT includes a 1x1 transparent PNG, located at modules/monks-active-tiles/img/1x1.png.

Personally, I don't recommend this. You will eventually lose track of where your tiles are. -Crow

Lower the Opacity

Give the tile any image, then go into the tile configuration window and move the tile opacity slider to 0. This requires the tile to have an image assigned to it; core Foundry will not hide it as the grey box is intended to remind the GM to give the tile an image as that is their intended purpose.

I also don't recommend this. -Crow

Use Tile-Z

This last one is the method I recommend the most. -Crow

In my setups, I don't even use the scene background, I just make a big tile that acts as the background. This allows you to set any tiles you want hidden to a lower Z-index than the background tile; you can then just send the background tile to a lower Z than those ones to view them without having to worry about losing them in the future, then set it back to the higher Z to hide them again.