It's not a question of presumption or acceptance.
The power to search (clause 90) operates no matter what one person or another may think of it.
Further, the intention or otherwise of a person to seek entry further into the complex is also irrelevant.
Their mere presence on prison land (even in the car park, and even , say "waiting while somebody else goes in")
is sufficient for a person to be liable to a search.
It's a statutory (well, regulatory) power.
Don't confuse it with, say, searching a bag in a department store as a condition of entry.