Good ideas, but I've found a few no-see-ums flat makers in tires before that couldn't readily be felt and only appeared as the tire casing flexed. Could be a small wedge shaped piece of glass, or a very small wire, or a skinny thorn needle.

Buy a new tube, then before you install the new tube take out your old tube and mark it so you know how it came out. Then take the old tube and with it outside the tire greatly overinflate it, like to where you get a bulge in it (you're not going to re-use it anyway at this point). You should now have enough pressure in the tube to let even the smallest leak show where it is. You can now use either the water trick, or place it up next to your ear and cheek to feel for any air escaping. When you find the leak then align the leaky tube up with the tire and rim to find the mysterious leak maker.

Also, when you say flat, how flat is flat? Does it just loose 20Lbs of pressure so it looks flat while you are sitting on the bike, or does it loose most of the air so it looks flat with just the weight of the bike sitting on it?