Salam alaikum

People at old age might be in status of dementia and probably remembering things that were in the past and feel that they see it. If you have looked multiple times and did not find anything most likely it is that your grandpa is imagining things.

If it makes him feel better maybe you can read the Quran for him. Next to that we have a Hadith that you make du`aa’ for Allah to clear the house from snakes by an actual call three times to the snakes to leave houses.