Try this:
blurredImage = conv2(single(grayImage), ones(3)/9);
imshow(blurredImage, []);
% Find where the image was all 1's in a 3x3 box.
% The output image will be exactly 1 in that situation.
matchingRegions = abs(blurredImage - 1) < 0.0001;
% Use axes() or figure() to switch to a new axes if you want.
imshow(matchingRegions, []);
You might also want to look at normxcorr2(). I have a demo for that if you want it.
