Both Hospitalisation and returning back to the home can cause a cat considerable distress. During a stay at the veterinary practice cats are required to stay in a cage, are often not able to hide, are manipulated and restrained by strangers and forced to share an environment with many other pets, including dogs.
In addition to dealing with this stressful new environment, depending on the reason for the stay in the vet clinic, the cat may be in pain or feel unwell which adds to the distress.
Once the cat returns home, things may not improve. While the cat has been at the vets, his reassuring marks may have disappeared from the walls and furniture and/or been replaced by those of the other cats. As a result, when he comes back home, even if the cat “visually” recognises his territory, without the facial marks, he will feel lost. Similarly, other cats in the household may not 'recognise' him, as he may smell and/or look different (due to bandages, elizabethan collar, etc...).