ABS Light puzzle

Discovered my ABS dash light (45D) had been disconnected by the previous owner by cutting through a wire to the dash.

Reconnected it and all seemed fine - it goes through the test sequence as expected.

It also passed the MOT last Monday, no problems.

I now realise that the light does comes on (and stays on) for some journeys. The strangest thing is that it never comes on when driving to work in the morning but nearly always does on the way home 9 hours later (17 miles each way).

How does it know the difference?

I haven't looked yet but presumably a sensor?

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