I think some of it depends on whether it's a gradual or sudden change.
My DS never slept more than 45 minutes at a stretch, he had severe reflux and couldn't tolerate more than a little bit in his stomach at once and so he was constantly waking up. Then one night he slept 13 hours at a stretch. At first I was thrilled, but when I woke him up he fed briefly and fell right back to sleep -- I knew something was wrong and took him to the doctor.
It turned out he'd had a reflux triggered asthma attack and his breathing was so off that he wasn't able to do the suck/swallow/breathe thing, sucking was exhausting him to the point where he fell asleep early in the feeding, and he had gotten so dehydrated that he wasnt waking up, which meant he was eating less. Other than the fact he was feeding less frequently, a slightly elevated breathing rate, which I'm afraid to say I didn't notice, some decreased breath sounds which were only audible on a stethoscope, and a depressed soft spot you couldn't see under his curly hair, the sleeping was the only symptom -- no fever, no coughing, no wheezing at all.
So, while I wouldn't worry if a healthy infant started stretching out their sleep -- or started missing a single night feeding at a time, I would worry about a sudden dramatic change (45 minutes to 13 hours certainly qualifies as dramatic).