Find and change all layers

@Steve – check the variable visible for all layers in the document, store some information about that layers, set visibility of all layers to true, search, replace, set back the original visibility of all layers.

Like that with storing the unique ID for invisible layers in an array:

//Temporarily make all non-visible layers visible
//Search and replace
//Set the visibility of all layers back again

//If you want to access locked layers as well, expand this example.

//1. Store all IDs of all hidden layers in array:
var myDoc = app.documents[0];
var allHiddenLayersID = new Array();
var allLayers = myDoc.layers;

for(var n=0;n<allLayers.length;n++){
    if(!allLayers.visible){allHiddenLayersID.push(allLayers.id)};
    };

//2. Set all layers of document to visible:
myDoc.layers.everyItem().visible = true;

//3. Do your search/replace

app.findGrepPreferences.findWhat = "\\s\\s+  ";
app.changeGrepPreferences.changeTo = " ";

app.changeGrep();

app.findGrepPreferences = app.changeGrepPreferences = null;

//4. Change all formerly hidden layers back to visible = false:
for(var n=0;n<allHiddenLayersID.length;n++){
    myDoc.layers.itemByID(allHiddenLayersID).visible = false;
    };

Locked layers will not be searched.

If you want that, you could expand this example with a second array for locked is true.

Uwe