What you've described is exactly what our 16 year old cat was doing. It started last year when she suddenly went blind (vet couldn't explain how). We also noticed that sometimes she would sit still with her mouth open, almost like she was in pain, and drooling. She started losing weight, but like your cat, continued to eat normally, could use her litter tray with no problem, and could still find her way around the house and garden (the fact we've been here 14 years probably helped). However, she would often yowl loudly in the garden like she was lost, but could always find her way back inside. She would also yowl loudly for several minutes when going into her bed for a nap during the day. The vet thought she probably did have dementia. For the weight loss, the vet kept saying that they could do more tests to find out what might be wrong, but in the end we decided that she was old, had had a good life, but the outlook wasn't good for her. So, sadly, we put her to sleep. But, we know we did the right thing for her in the end. We think she may have suffered a stroke at some point but couldn't be sure. What ever you decide to do with your cat, I'm sure you'll do what's best for you and them.