My car is a 2007 model FIAT Albea 1.3 liter 4-door saloon. I think it might be the "fire" version. Made (assembled) in Turkey. I use it 6 months a year but 4-5 weeks at a time. When it's stored/parked at my friend's place he keeps the engine running but does not drive the car. Invariably he has to replace a wheel, put on spare, take the flat tyre to a repair shop only to be told that there's no puncture! The tire goes flat "for no reason" but why?
Your wheel is obviously leaking somewhere.
There are a few places that you can have a leak related to the wheel
The Schrader Valve - The schrader valve within the valve stem where you refill the air can be bad and require replacement.
The valve stem - The valve stem where the schrader valve is in can have a bad a seal on the rim
Tire Pressure Sensor - I didn't look up if you have one but many cars do have a tire pressure sensor. You can have a bad seal on your wheel.
Tire Bead - All around your tire where the inside diameter of it meets your wheel requires a nice smooth surface. You can have leaks here.
Any damage to the surface of any of the above points can cause a leak. It could have been a tire change from a careless tire worker. A good scratch along the bead, a pair of pliers replacing the valve stem and a nick in the valve stem hole.
You may need a new rim but you would have to pull the wheel and remove the tire, valve stem and pressure sensor to give it a good inspection to determine where the leak is and see if you can fix it with some sandpaper if it's a bur on a sealing surface of your wheel.
One way to determine where the tire / wheel is leaking, is to make a solution of soapy water and put it in a spray bottle. Take the wheel off the car, and coat the tire and wheel on every surface with the soapy water. The place where the bubbles are active is your leak. I have no doubt that one of the places @DucatiKiller mentioned will be the problem.
Hope that helps!
Two issues here:
The wheel rims might be rusted or encrusted with dirt, which prevents a good seal between the wheel and the tire. In that case, the tires should be taken off and the wheel rims carefully cleaned, de-rusted and sanded smooth. The tires should then be mounted and sealed to the rim. There are various techniques for this, each with their own pros and cons. Google.
Tires are filled with air under pressure. Air consists of about 78% nitrogen, 20% oxygen and a few other gasses. Oxygen molecules are small enough that they can actually pass through rubber -- slowly, but steadily. In addition, oxygen is very reactive -- it will corrode anything it can. Then there's also the moisture in air, which will hasten any corrosion process. Airplanes and racing cars therefor have tires filled with nitrogen. You will need to find a service station that has pressurized nitrogen. The nitrogen molecule-pairs (N2) are too big to pass through the tire walls (unless the rubber is falling apart, of course) and it does not contain moisture. In addition, pure nitrogen expands and contracts less than air does in the usual temperature ranges of most climates. I had my tires deflated and filled with nitrogen at a service shop that does oil changes and the like. It cost only 16 euros (4 euro per tire).