Hello - which led stays on please? Not aware of any.
On my devices, the LEDs only light up when the settings are changed through the app or on the device.
Scheduled temp changes do not cause the units to light up.
There is a device option called Dazzle Mode that says "Your Tado device shows an animation when settings are changed via Manual Control". This is on by default so you could try turning that off
I have some home automation, that also causes the light to turn on
Examples: doors open, turn on hot water etc
Sorry I found it! Go to Rooms (not the device!) and in the properties you can change 'Dazzle Mode' (Dutch: Schittermodus) and disable it