Roof leaks are the obvious one. Salt air and summer storms wear down flashing, shingles, and vent boots faster here than they do further inland, and a small leak can drip along a rafter for weeks before it ever reaches the ceiling drywall below.
Condensation is less obvious. An attic AC unit or ductwork running through a hot, humid attic space sweats, and that moisture drips onto the insulation and ceiling material underneath. It shows up as a stain with no obvious roof problem, which is often the first clue we're dealing with condensation rather than a leak.
Upstairs bathrooms are their own category. A worn wax ring under a toilet, a cracked shower pan, or grout that's failed can send water into the floor and straight down into the ceiling of the room below. If the stain sits directly under a bathroom, that's usually where we look first.
A color change alone doesn't tell you which cause you're dealing with, which is why we check all three before recommending anything. A brown ring points to standing water at some point; a yellow-gray haze without a clear edge often points to condensation instead.