Even if the plods on the ground making the initial arrest couldn't determine the difference between 'noise' and an encryption, there would be others who surely could examine the computer and determine the facts.
No, this is simply not possible.
(it sounds like) You're thinking of a computer file as a readable object, kind of like weak encryption; such as a .ZIP file with a password or some such.
No, what the others are talking about is things like hidden archives, and 'typeless' files.
The easier of the two to explain, is the typeless file; it's just a bulk of 'noise', there's no file type. No common code. No... well... anything, to discern it from an encrypted file, or a 'scrambled memory dump' you generated to waste HDD space on purpose (Sounds stupid, but there have been many times in my life ive needed to do this. Disk burn-in, for example).
The harder one to explain is the invisible archive. There is no file, there is no.... anything.
To the Operating system, it looks like blank disk space. You just have to be careful not to fill said blank space, because it's not blank.
Say I have the drive sitting in a cupboard for 15 years, and it gathers a LOT of solar radiation over the years, and gets the entire drive surface scrambled (this isn't uncommon);
There is no way to know a 'broken drive' from a 'hidden archive'.
Then you get fringe cases, where a cracker might insert "secret file.zip" into a hacked installer for, I don't know, VLC player.
You install VLC player, the protected ZIP gets dropped, you know nothing of it.
Then, they find it, and since you can't provide this password, you go to jail.
Unlikely? Sure; impossible? far from.
Especially in the days of sharing a computer, and backdoor trojans.
If you got infected and didn't know it, you could have a hidden archive on your 'blank space' right now, you'd never know!
nothing can detect it. You need to do electron analysis; or at least low level block stuff. No 'app' will find it.
In fact.... on their mobile phones, MILLIONS do!
This is reality!
Mainly from cracked APK's.
They're impossible for the user to remove without a format (and even then it sometimes needs a firmware rewrite!)
Just a little encrypted session for data theft, bot work, and god knows what else the user DOESN'T want.
You can't jail all those people for something they don't know.
Hell, android 6.0 and above encrypts the entire phone, including the ROOT partition. I don't know that password! It's hashed. No one does.
Providing my phone password won't help. Even if the person is willing.