Why is it leaking?

A NEW RECORD FOR LONGEST 1ST POST.
2004 Camry XLE 4 cylinder 207 thousand miles
guess at 2.4L engine. but there are 3 engines offered.
2AZ-FE ENGINE SURE.
THAT OVERHEATS,
LET ME CONDENSE YOUR POST.
1: NEW LEAKING CAB HEATER CORE.
And sure after it stinks how can it not 50%AF toxic fluid in the passengers foot well carpets. gee get them steam cleaned by pro's (removed and pro cleaned)(

2 :now the coolant is leaking and spraying in the engine bay well that is normal on all cars this old (not serviced, abused)
all hoses bad and 19 years old cracked and leak what's new there, zero..nada.

or water pump leaks
or radiator leaks, sure, 19 years old sure it can.
nothing new there.
what will be ,will be. there is no magic to car repair
--------------------- NOW COMMENTARY BLIND------------
best guess:
he used bar's leaks to stop tiny leaks. that is all it does
it never ever fixed cracked hoses ,bad water pumps nor fixes radiator leaks. or bad head gaskets or warped heads.

if he was a pro he;d have told you the car needs major service, and parts but when told by him, the wallet was dry
outcomes snake oil BAR's leaks.
fix it right or patch it and pray. <<<< your CALL !

BAR'S: (MAKERS REAL NAME)
it can help some times in pinch 1000 miles from home
it's a band-Aid to a broken leg. ( not magic, it is a desperate act)<, NO BLAME ME IT IS YOUR CAR.

old cars are all like this they have vast problems and most ive seen 10 problems ignored by the owner.past and present. (all) they add up, and then the camels back is broken with 1 more gain if straw.!
machines need service, if not they fail or fail hard.

if the engine is allowed to overheat too badly the engine is wrecked, ever price out cracked heads and engine blocks.
some cars heater cores are $2000 fix, dash out 12hours labor.+parts.
seems to me you got a SUPER DEAL there.
MY GUESS JUST HEATER COR HOSES BAD
AND ARE THE HOTTEST HOSES AND WHY.

Each leak must be found and if hoses crack they are all bad, for sure if 19 years old.
the RAD tank ends can crack TOO.
and the pump fail and the path of the whole coolant loop can fail and crack a real pro finds them all.
even intake manifolds can leak or thermostat housing can crack, SEE????????
TOYOTA Camry 04
lot to fail on any cooling system
even rad fans.
WE CHECK AND CURE ALL LEAKS FIRST
UNDER FULL HOT ENGINE PRESSURE,NEAR 15PSI
FOR LEAKS, PARKED HOOD UP EVEN RUN FOR 30 MINUTES TO FIND THEM ALL
CAREFUL AND COMPLETE OBSERVATION,
FOR SURE DRIPS OR TRICKY SQUIRTING OF 50%AF COOLANT.
NO 19YEAR OLD THERMOSTAT IS GOOD
AND IF BAD AND CAUSES OVERHEATING THE SIDE TANK NEXT TO RAD EXPLODES IN STEAM AND IS SUPER BAD FOR ANY ENGINE (JUST BEGGING TO WARP THE HEAD)
AND A NEW SERP BELT TO THE PUMP IS BEST.
THIS OLD.

If you are talking about the reservoir that hose is the overflow hose.
Its possible the shop did not release all of the air pockets from the system and some coolant came out with the air bubbles.
The tank should be about half full hot and cold.

Check the transmission fluid level. Sounds like you either have a leak n or failed to properly top off the fluid level after the initial loss of fluid.

Check for blown head gasket if there is steam coming from the exhaust,also check that the theremostat is opening.

check top radiator hose goes to thermostat housing sound like its leaking if your engine leaking from left driver side on top of engine it has to be top radiator thermostat hose leaking.wait until engine cool down replace top radiator hose see if coolant leaking still occur if so you could have radiator leak.

check water pump

it seems to me your car lost coolant through a crack in the radiator or from the cooling system hoses, which resulted in it overheating. what you can do is pressure test the cooling system to pinpoint the source of the leak and rectify it. it could be anything from a damaged radiator, worn hoses to loose hose clamps. after doing that check to see if the fans are working and if so take your car for a spin. the car should be ok. if during testing the temp goes up then you need a new cylinder head gasket and it could be a bit expensive.

This one is not easy to tell without seeing the car. But, a logical guess would be that the water pump is starting to go bad and is leaking intermediatly. If you can pressure check the cooling system or have it done that should give you the answer.

there are many can cause the anti-freeze to leak.
you need to check to see if the leak come from the hose, water pump, gasket or radiator.
if it leak in the front then the radiator or hoses
if it leak on the passenger's side the it can be water pump or heater hose.
make sure you have coolant before you drive, because it can over heat and ruin the engine

The light is letting you know that the engine is oveheating. You did not mention how many miles are on the car, so the over heating could be a number of factors; Thermostat is stuck closed, cooling fan is not operating, radiator is leaking...to name a few.