Part of me thinks you’re now overthinking it and assuming the worst where it may not be needed. Sure they may get in touch occasionally to check the vehicle is being used in line with the scheme requirements, but I would expect them to ASK as opposed to just accuse based purely on vehicle location without additional proof.
I would imagine a conversation along the lines of:
Motability: We’re calling to check on the use of the vehicle, it hasn’t been to the lease-holders address in the last 10 days and appears to be nearly 150 miles from the address.
You: we’re on holiday, and the lease holder is with me
I would expect a reasonable explanation would end the conversation very quickly. If the car was away from the house for weeks and regularly, they might ask for more evidence but one-off unusual items I would think would be almost expected.
"Man is born in freedom, but he soon becomes a slave, in cages of convention, from the cradle, to the grave."