Re: Sensors in a Shabbos elevator – the way the Shabbos elevators I’m familiar with work is that the doors remain open at each floor for a given amount of time, after which a buzzer sounds for a few seconds, and then the doors close. It was explained to me that the sensor is de-activated when the door is open but the buzzer is not sounding – and part of the reason for the buzzer is to let people know that the sensor is active and they should not pass through. That way, there is no Melacha issue (since the sensor is inactive when people get on and off), and no safety issue (since the doors will not try to close when there is no active sensor).
Re: escalators in general on Shabbos – I can’t speak to Shabbos, but I’ve had two situations where I was told that it was not an issue to use an escalator on Yom Tov. These, though, may not be germane to the issue at hand as 1) using an electric current that is already on on Yom Tov for something that is a “davar hashaveh l’kol nefesh” is generally mutar, even if it causes an increase in the current; and 2) once was when it was Yom Tov Sheni and I was in Chutz LaAretz, so it wasn’t actually even Yom Tov for me – though in public (and according to many/most poskim, even in private), I was not allowed to do Melacha.
an Israeli Yid