2459. A cat lost 4 days, will not eat - traumatic peritonitis?

A 3-year-old male neutered cat disappeared for 4 days. He came home not eating and weak.  
Cloudy abdominal fluid could be blood. The cat was severely dehydrated but is now eating 2 days after the IV drips. The first vet examined him to be severely dehydrated and the bladder was tense on palpation. His blood test showed high serum urea. The cat had fever of 40C. The cat was warded 3 days and given antibiotic injection but the vet was not able to ascertain the cause of the high fever.

The owner took the cat to Toa Payoh Vets. I asked permission for an abdominal X-ray. Surprisingly, it showed radio-dense peritoneum ("X "on X-ray images).  A haemo-peritoneum where there are blood clots blanketing the intestines such that they could not be visible as in normal cases. This bleeding could be due to the cat falling down from a high rise or thrown or kicked in the stomach area?

Sometimes an X-ray is deemed not necessary since the cat is not lame.  After 2 days of IV drip, the cat eats the canned food provided by the owner. He is still defensive but not wildly ferocious.