I’ve certainly not found one that won’t cover a bike being locked to an immovable object
Whilst I’m not sure most would cover this it’s not especially unusual (I had a discussion with esure about bikes *in* a car last year at renewal, I didn’t like the answer)
such as a bike rack locked to a car.
Given how easy a bike carrier is to remove I’d certainly want to check if they considered that to be immovable.
From asking at renewal times, internally in a locked car, seat rails counted with some, others didn’t consider then immovable as you could take the seats out.
All that being said, policies, underwriters, exclusions and inclusions change noticeably year to year.
The big thing is check what’s in your policy when the renewal comes up, just because last week it was covered for someone else is not an indication it is in your policy.