Coolant leaking in my 2001 911 c4

White smoke may indicate an internal coolant leak or low coolant reservoir levels. Coolant leaks can also leak into the oil of your car causing it to be frothy or milky looking smoke. You might have a cracked head gasket, engine block, or cylinder head which are all caused by overheating.

the tank might have a crack or hose leak if not tank the heater hose lines run under there also

Only if you have to remove them to fix the leak. Make sure the leak isn't coming from the water pump, the reservoir, or from freeze plugs.
If it has overheated or is shooting water out of the tailpipe upon startup, you should have a mechanic do a pressure check.

So the level keeps dropping in the reservoir? If you have to keep adding coolant to the reservoir, the system is leaking or losing coolant somewhere. Have the system pressure tested to find the leak. Is the reservoir itself leaking? When the level gets low enough, first the reservoir will go dry, then the radiator level will start to drop.
Here's how the reservoir works: coolant is under pressure from the radiator cap seal. As coolant heats up from hot engine, it expands, and pressure forces the radiator cap seal up, coolant is forced into the reservoir-remember the hot mark on the reservoir? When car is shut off, coolant contracts, creating a vacuum in top of radiator. This vacuum works to suction coolant back from reservoir into the radiator, keeping the radiator topped up, and coolant should now be at the lower cold mark on the reservoir. It is a closed, sealed system. The only loss of coolant will be a very small amount of evaporation from the reservoir. If coolant level keeps dropping, there is a leak somewhere.

my '01 Jetta had a similar problem. I could always smell coolant and the bottle was always low. I crawled under the car and there it was..a caked on mess of pink antifreeze. There is a plastic intersection 3 way valve where the coolant temp sensor resides. it has 2 bolts and a rubber o-ring. Because it is plastic, it cracks. it drips down onto areas which are really not visible from above and then sizzles and melts onto areas under the engine. The part was only about $25, took 30 min to install and my problem is now fixed. Air was getting into the system through the small crack in the valve further complicating the system operation.

it is getting sucked into the engine through a crack or bad gasket.
needs attention ASAP. take a sniff at the tailpipe for antifreeze odor, or look for foam on the bottom of the oil fill cap. ain't good.

are you sure the reservoir is not cracked or? because as you drive the engine coolant expands as you drive and collects in the reservoir, then after you shut the engine down the coolant cools and contracts creating a vacumn and it goes back to the radiator as needed. If the reservoir is cracked it will after you shut it off as the coolant will be higher.

head gasket is blown

Your problem is something like either a hose clip loose /biting into a hose and split it/a metal hose rusted and has a pin hole or a welsh plug (core plug) leaking. or a water plate gasket IE W/Pump/Manifold etc
Because it drips of the centre of engine dosenot mean it coming from the bottom of engine...

i had a hairline crack in recovery tank on bottom side ,very hard to see.this on 3.5 high output. also if you have had 2 add coolant you probably got air in system, you have to use bleeder on top front of eng to get air out.if you cant find leak put piece of cardboard under eng run then shut off let cool then ck cardboard 4 signs of leak.also ck tailpipe 4 signs of coolant, if headgasket has leak it could b leakn in cyl and burnin out of exh.