yekke2 –
With regard to the case of the light in front of the house: while a sensor may be a switch, min hastam you have no idea which step will cause the light to go on/off. Therefore each step is a davar she-eino miskaven, which is mutar, because every davar she-eino miskaven that does not inevitably cause a melacha is mutar. This isn’t only my logic; I have heard this from many rabbanim.
With regard to the toilet case, there are a number of factors to consider, including: 1) It is kil’achar yad (probably what Sam meant), since you are simply getting up to go on your way. While R’ Shlomo Zalman was known to hold that if this is the way it is always done it is not called kil’achar yad or grama, this is a chiddush which others reject. 2) R’ Shlomo Zalman himself would undoubtedly maintain that the electronic mechanism that makes the toilet flush is not d’oraisa. Therefore one can apply principles such as kevod habriyos, tza’ar, oneg Shabbos, etc. to say that one is not obligated to hold it in or stay in the bathroom all Shabbos.