I really wonder, what do they do if you pretend that you forgot keys? I think that they might give up on a very average person but if there is a guy with 10k and higher number of bitcoins in front of them, what do they do?
That's what the 5 years in prison is for.
I have crossed borders with Electrum installed on my smartphone and PC but no one has ever asked me anything.
If no one knows, that means that it didn't happen, which means no need to declare anything.
That's the wrong approach. I always declare everything I have to, up to an apple in some countries. If it's not allowed, I'll throw it away. But at least I won't get into any kind of trouble.
For Bitcoin, this is my reasoning:
You could argue you're not bringing anything physical. Otherwise hiding it from customs would be illegal.
Bitcoin isn't in your wallet, Bitcoin is on the blockchain.
You should probably not bring a copy of the blockchain holding
19,774,912 BTC if you use this argument

Seriously though, it's like bringing a key to a safe deposit box. You don't have to declare that key.