Sealing an inaccessible hole in a masonry foundation

Question

I just discovered I have small holes behind my water tank in a tight closet inside my 2nd floor condo. I was vacuuming around the water tank and something like a thin rope was sticking out of the corner, so I thought it was just piece of something but then it disappeared inside the corner. I checked today, 3 days later and there appears small holes that weren't there before. Is it a lizard? A mouse? A Snake? Son't see any droppings though. It got unusually cold outside, may be "something" found it way inside attracted by the water tank warmth? But it is on the 2nd floor! The problem is I don't know what to do. I am extremely terrified of fast moving creatures. Secondly it is almost impossible to reach the spot to seal it. Lastly, I found my extra pillows that were sitting on the shelf in a closet in a different room on a floor. Could rodents, if it is rodents run around and climb the shelves on that (different) closet. There is no way they would just fall down on its own. So I am terrified now.