Constant flat tires

My wife and I bought a new X3 not even one year ago.
The first few weeks that we had the vehicle my wife drove it aprox 30kms to work.
The flat tire warning light came on very early on the trip, probably within 10kms
She continued to slowly make her way to work
Arriving at work she discovered that all 4 tires were flat and 3 of the were completely flat!
( Did not even register on the tire gauge)I went to her work place and put enough air in the tires to be able to drive it up onto the flat deck truck that BMW sent out to pick it up.
Of course the dealership had never seen anything like this before!
The dealership put a thick sealant on the rims to help seal the tires better
I was concerned about the condition of the tires after been run totaly flat, but the dealership told me that there should be no concerns!
The vehicle would have had less than 1000kms on at the time
Now the vehicle is having trouble holding air on the left rear.... with aprox 26,000 kms on the vehicle.
I do like the dealership in Saskatoon, BUT when I talked to one of the service writers, he asked me how often I checked the pressure in the tires. I told him that I hadn't for some time. He advised that I should check the tires on a regular basis!
This is when I got upset. This is a vehicle for my wife. I told the sevice writer that I seldom have anything to do with the vehicle and like it that way
I told him that I like to check the oil in my vehicles engines myself too, but BMW says it is not necessary for me to do that task, so why should I have to keep checking the pressure in these flippen tires?
Has anyone out there had anything similar happen to them, and what have they done about it?
Sorry for being so long, but I believe the dealership should have replaced these tires when we first had all of the tires go flat?
Left rear was 10 psi low tonight... After about one week

Can only speculate on what caused original problem.

But losing 10 pounds in a week in a run flat is typical of a slow leak, typically a nail or screw still stuck in the tire. I've had mixed results with getting this fixed. Some dealers will plug/patch a puncture in a run flat, others won't and will only replace the tire.

As far as checking tire pressure regularly the TPMS does this. And you'll notice that when the outside temperature drops 10-15 degrees, the TPMS will come on identifying some or all of the tires as low. In the old days when cars went in for service every 3000 miles, the tire pressure would get checked regularly. Now with extended service intervals, it's not getting checked as often. While annoying at times (it always seems to come on when you're running late for an appointment or the weather is cold and miserable), the TPMS is a good thing.

With large temperature swings deflated tires then to occur more often. Any chance that you have a bead problem?

Yes a bead problem, that's why they doped up the rims with some type of sealant.
Never seem to have any issues in the summer. It's when the freezing temps come our way

OP, I am not sure if sealant will fix that problem. I think they should have removed the tires, cleaned the rims, then remount them.