There will be no proof of an operating system existing at all, just random data. If you use VeraCrypt along with a hidden partition normally, you would still have the VeraCrypt bootloader or an apparent Windows installation on the drive.
But back to your question, truecrypt was professionally audited and deemed “secure”, some issues were found but none that were back doors or significant. Shortly after(might have even been during) the audit truecrypt deleted all old versions and posted a weird message telling people to use bitlocker.
After some time veracrypt picked up the torch and has continued developing what was truecrypt.
https://en.wikipedia.org/wiki/Paul_Le_Roux
This guy's life definitely needs a movie
The possibility of being Satoshi of bitcoin also. Entertaining read. Programmer -> cartel boss.