Hi Koigal,
if I understand you correctly, you have random leaks while standing, sitting and walking.
I think that these leaks are not as "random" as it may seem. In fact, it is most likely part of the stress incontinence problem. The crucial factor is the pressure in the lower abdomen, which increases during such activities. Normally, one already unconsciously tries to work against this with the pelvic floor. If the pelvic floor or sphincter is weak, however, this is sometimes no longer sufficient.
This is sometimes difficult to reproduce when it is in a "border region", so to speak. For example, if you lift something on purpose to stand up quickly or climb stairs to increase the pressure then you think about it, tense the pelvic floor and nothing happens. It only happens when you don't think about it and the normal tension is no longer sufficient.
If I understood you correctly, you have already been to two PT's and they could not help. How long and how often did you do the exercises? Normally you should do it daily - or even several times a day - even if it gets better.
The pelvic floor is basically a muscle like any other. If you go to the fitness center twice, you won't come out as a bodybuilder. It's the same with the pelvic floor - it takes a while.