What does "betraying the fact" mean?

Question

I'm reading a BBC article on ketamine abuse. In the article it says:

The doors at the Baiyun drug rehabilitation clinic are always locked, betraying the fact that the patients inside aren't always there by choice.

It doesn't make sense to me that locked doors at a clinic betray the fact the patients aren't there by choice. I am from the United States and I know it's possible the United Kingdom may have some alternate meaning I'm not aware of. Or maybe I just don't understand the word's application here? Thanks

Answer 1

The sentence is telling us that some patients are there involuntarily—confined, locked in—and that that facility would or should be reluctant to confess or reveal that such is the case.

This sense of the verb betray is perfectly at home in America too, per Merriam-Webster's definition:

4 a : to reveal unintentionally

OED lists it too:

5. To disclose or reveal with breach of faith (a secret, or that which should be kept secret).

Answer 2

To betray something can mean to reveal it to public view, usually not intentionally:

betray v tr
4. To make known unintentionally: Her hollow laugh betrayed her contempt for the idea.
[TFD]

So while the clinic mentioned may wish to pretend that people in its facility are not prisoners, the locked doors give away (betray) the truth of the matter.