They couldn’t know and neither you or they could prove there was more hidden data. The best part: you can even hide data in the hidden portion and then hide data in this section, and so on …
Now, everybody knows VeraCrypt (and TrueCrypt) has this feature so why should someone believe you didn’t make extensive use of it. You simply can’t prove that you haven’t given up all your passwords.
The problem with this: for example in the UK you can be jailed for withholding your encryption keys, even if you genuinely forgot your password(s) because how would/could you prove that.
As long as law enforcement thinks you’re withholding you’re basically f…ed.
XKCD said it all:
When would you stop using the wrench, knowing there is a “hidden container” feature?
It is actually a non-feature!
No! Well, you could copy your data into a encrypted system you set up later. You will not be able to convert your system in-place, though.
It is not that difficult to set up during the installation of EndeavourOS. If you don’t need anything fancy like logical volumes there is a checkbox (“Encrypt”) that will get you set up.
If you’re interested in a little more complex setup, read this.