Out of the blue large amt. H2O pooling in trunk. No obvious entry but the source is rain water.

Hunting water leaks is a pain in the tailbone job to do. I'm assuming yours has the jack in the floor of the trunk, under the carpet.
The spare wheel well is the lowest point in the trunk so the water will collect there regardless of where the leak is. Start with the simplest tools, a flash light and the palm of your hand.
How much water is collecting and how fast does it fill up? Rain water coming up through the floor is unlikely unless the floor is damaged.
First take everything out of the trunk that wasn't in there when it came out of the factory. Take a really good look at the carpet and trunk side liners. Use the palm of your hand and feel the floor carpet for wetness. Your feeling to see if it is wet in a particular area. Using the flashlight look for darker patches as carpets often darken in color when wet. Move to the rear of the trunk and feel the liner on the back of the rear seats. Next run your hand over the side panels where the wheel arches are. This will give you clues as to where the leak is possibly coming from.
If the whole trunk carpet and liners are bone dry the leak is from under the car.
All the doors, the trunk lid and the sun roof are fitted with rubber seals to stop rain and road splashes getting inside the vehicle. Over time these perish and start to leak. Run your hand around the underside of the rubber seal the trunk lid sits on when closed. This is the most obvious place it will leak from. You can do this on each door as well because water can travel a large distance before it finds somewhere to create a puddle. A Sunroof can also leak but they tend to cause puddles in the foot well.
If you get a few dry days you can test for leaks with a hose pipe. Keep the trunk closed and make sure the inside of it is totally dry. Remove any spray nozzle from the hose so it flows at regular pressure. Carefully aim the water so it only flows over the trunk lid. Let it flow for a minute or two trying not to soak the doors at the same time. Turn off the water and leave the car alone for about 10 minutes. Wipe off any excess water on the trunk lid, open it and see if its now wet inside the trunk. Yes? the trunk seal is leaking. You can use this method for the doors and sun roof as well. Door and trunk seals are usually just a 'Push on/Pull off' fit so you can easily change them yourself.

How wet is the foot well? This is likely to be one of two things. Door seal rubber or the Heater matrix.
Try this bit of detective work. Does it only get wet when it rains heavily? Is it just the front that is wet? Feel the carpet in the foot well. How high up the foot well carpet does the wetness go? Does the wetness smell of anything? Do you have a sun roof fitted? Sun roofs have drain holes that can clog up so the rain pools up in the grooves and runners then over spills inside the vehicle.
If it only gets wet after rain it could be a worn out door seal rubber or the sun roof letting water in. Door seals usually cause shallow pools of water so the wetness won't rise very high up the carpet. The sun roof sometimes causes the seats to get wet as well.
If it is wet all the time, the wetness rises high up into the foot well and it has a 'sweet' smell, it could be the small radiator unit,(the Heater Matrix), behind the dashboard that supplies heat to the heater blowers leaking. Depending on where exactly it is fitted you'll sometimes get wetness in both front foot wells. The smell is the coolant from the engine.
Hope this helps.

hi i'm not familar with your car but if you can wind the sunroof back so the sunroof is fully open look at all four corners where the sunroof sits into there should be at least 2 drain holes which can over time become clogged with debris, if you find them use some warm water and flexi pipe to clean the drain holes out. hope this helps

im gonna suspect that an 07 has got to have some worn out rubber on it , im gonna suggest you source a new set of gaskets for the sun roof , i;ll bet that will help

Hi.
I was having trouble with my sun roof on a 2004 Montecarlo. I had two issues water coming in and would not operate fully. I could toggle the switch and it would not move very far. this is what I found. I started to pull down the lining and saw that the motor was in front of the sun roof.
I found the drain lines on the front side corners of the tray around the inside of the sun roof. I found that the tube ran down the pillar's and out between the door hinge's. I took them off and blow them out one was plugged up. That fixed the water problem. The roof would move with help from me pushing. I got it opened and noticed the grease was hard. I cleaned up the grease and removed most of it with WD40.
Now the sun roof works good as new.
I thought the water problem was because the roof was not fully closed but I guess it is designed to have water pass by the roof seal and have it drain away. I always noticed water under the car after a rain or a car wash but did not realize iit was the sun roof draining.

Sun roof drains are clogged

Sun roof drains are clogged, clean them out

You need to remove the head liner
If all the hoses are attached,then squirt water into them with a small bottle and then blow them clean with an air hose,check on ground for water

Not familiar with the Volvo, but many cars (Toyota, Lexus, Acura, Honda, etc.) have drain holes at the corners of the sun roof assembly. These holes allow water that gets past the sunroof gasket to drain through tubes that carry the water to the bottom of the car, where it exits the car.

These drain holes are prone to clogging. If the Volvo setup is like those cars mentioned, you can see the front drain holes by opening the sunroof and looking closely at the two front corners of the assembly that holds the sunroof. I have used plastic weedeater line to open up these lines. Compressed air can also be used.

you need to check and see if there is any debris blocking the drain tubes for the sun roof. If not then you need to follow the tubes and look for kinks or other reasons why its not able to drain off quickly.