My wife has the same problem on her Ford Escape. One time she got a nail in the tire. Removed the nail and plugged that. 6 months later both front tires started to lose air pressure. This time I found the valve stem rubber was cracked. I replaced both valve stems. Now they've both started leaking again. This time it's leaking the bead is leaking around the magnesium rims so I have to dismount them, clean the rims and put liquid tire seal on the rims before remounting and balancing. Fortunately I have the tools to do this.
I suspect since your tires have a slow leak they are also leaking around the bead. The soapy water will find it. You may have to look close to see the tiny bubbles.