Good on you Beeman for checking your pressures and avoiding a tire failure on the highway!
I had a leak of about that rate of pressure loss on a minivan and it showed up clearly on the old soap bubble test. I took it to the local tire sop and paid $29 for fixing a tiny roofing staple hole. The fix failed and I kept driving on it for weeks (adding air every week) until convenient to take it to Costco where I bought it. There was a warranty with free puncture repair but the tire man refused to fix it because the outside of the tread had worn too low - probably due to driving with low pressure.
Use a Mixture of water and dish detergent in a spray bottle, just pick a tire and spray until you see bubbles. I agree, test the valve stem and valve first. The olde valve test is to spit on a finger and rub it over the stem end. VERY odd it is happening simultaneously to four wheels. Has to be a valve, stem or tire defect. Do you happen to have those recalled Michelin tires? That would be good news as you get free new tires!