I believe you.
Some years back I had a cat who lived largely outdoors, coming home only for food (several times a day) and when injured or sick (apparently knew that the sucker who bought him an endless supply of food would also take him to the vet and pay the bill). One night he turned up in "his" chair (yes, he regarded it as his, despite occupying it rarely) looking quite under the weather, and showed no intention of going back outside. I felt around for signs of injury or abcess and found none. Took his temperature and found it to be very very high, so immediately schlepped him off to the emergency vet.
They searched and prodded for a bit without finding anything, but finally located the problem: a HUGE abcess, running under the skin/fat layer along an entire side of his body -- none of us had been able to detect a lump typical of an abscess, because it covered such a large area, and very evenly -- vet and techs said it was the biggest abcess they'd ever seen. It had obviously been festering for months to reach that size, and whatever original injury had started it had long since healed over. He had been acting perfectly healthy the day before, jumping to high walls to head off to neighboring properties, coming home to eat several times and immediately leaving again -- clearly his giant abcess had still been encapsulated the day before. With a fever of over 104, it had clearly become unencapsulated. Took a long surgery with 2 big incisions to clean it out, had to leave drains in for 2 weeks. He healed up, though, and enjoyed a few more years of making his daily rounds to neighboring homes and shops (he was very popular in the neighborhood, and both homes and shops often let him inside to visit).