Q: Car involuntarily accelerates 2-car lengths 1 out of 100 times

I purchased a Lexus-certified used hybrid car and it is exhibiting involuntary acceleration. Two to three times out of 20 when I start to brake, my car accelerates slightly on its own. My car has regenerative braking, so I thought that perhaps the slight voluntary acceleration was normal. It is startling when it happens, but I apply hard pressure to the brake and it stops the acceleration. But “every” time I brake, I have to keep focused and aware that it might happen. When it started doing this, I thought I was hitting both pedals at the same time, but I’m sure that I’m not doing that. The dangerous problem is the distance that it accelerates 1 in 100 times when I brake. My car will accelerate “two” car lengths before I can brake. Naturally, when the mechanic at the dealership rode with me, it didn’t happen! I was told to keep a watch on this and they recorded my complaint. This two-car length acceleration only occurs during summer hot months, like now. There has been no change in the frequency. Have you heard of this in other Lexus hybrids and can you advise me how I can get this fixed (before an accident happens)?