What flashing is doing
Flashing is the metal that turns water out where the tile has to stop: against a wall, around a pipe, along a valley, at a chimney, or around a skylight curb. Tile cannot be bent to fit those shapes. If the metal is short, rusted, or lapped backward, every rain uses that joint.
The pattern on Cerritos houses
South-facing pipe boots bake and split. Valleys under street trees fill with leaves. Two-story walls in Shadow Park and Sorrento leak at step flashing. Chimneys leak at counterflashing that has pulled out of the mortar. None of those show up as a hole in the middle of the roof.
The repair is on the roof flashing leak repair page: lift the tiles that cover the metal, replace the metal, relay the tiles.
Why caulk does not fix it
A bead of sealant on top of tile or on the face of the flashing cracks in one summer. Water then gets behind the sealant. A real repair is the lap of the metal and the tile course over it.
Questions people ask next
If the tile looks perfect, can it still be leaking? Yes. That is the normal flashing leak.
Do valleys count as flashing? Yes. Valley metal is flashing. It is also the first place debris collects.
How do I describe this on the phone? Say which room, whether it happens in rain, and whether there is a wall, chimney, skylight, or bathroom above the stain.