A door that was never built also says no
Page not found means both "you may not see this" and "there is no such thing". We nearly recorded a missing feature as a working security check, because from the outside they are the same answer.
We were checking whether one business could open another business's invoice. We asked for it and got not found. Good — that is what a working fence looks like.
Except there is no such screen. The address we were testing had never been built. Every request to it answers not found, for everybody, including the person who owns the invoice. We had not proved a fence. We had proved an absence, and the two are indistinguishable from the outside.
Not found is two answers wearing one coat
"Not found" is used for two completely different facts: this does not exist, and this exists and is not yours. The second is deliberate, and it is the right design — telling somebody they are forbidden from seeing a record confirms the record is real, which is half of what anyone fishing wanted to know.
But it means you cannot read anything into the refusal on its own. The only way to tell the two apart is to ask the same question as somebody who should get an answer. If they get it, your refusal means not yours. If they get the same wall, your refusal means not built.
Where this bites in a venue
A manager tells you they cannot see last month's wastage report. You check their permissions, find nothing wrong, and go round in circles — because the report they are describing does not exist, or exists under another name, or was never switched on for that site. Nobody is lying; the system said no and they reasonably read it as "not allowed".
Before you spend an afternoon on permissions, open the same screen yourself. One minute, and it splits the problem cleanly in half.
The habit worth keeping
Every negative result needs a positive control. Every "they can't" needs a matching "but I can". Otherwise you are reading meaning into silence, and silence is the one answer that fits every explanation — the fence working, the fence missing, the feature missing, the whole thing being down. We have made that mistake and written the check that stops us making it twice.