That can only happen if Bitcoin is banned in that country; if it's in a country where they publicly recognise the use and ownership of Bitcoin, then the person won't be in trouble unless the Bitcoin is linked to one of their blacklisted addresses, or if the holder can't explain how they came about it, that's where the real questioning will come from.
Even the country accept Bitcoin and you think your coins are not linked with blacklisted address/tainted coins, it doesn't mean you're free from trouble.
Government and tax officer want to earn as much as they can, if you show them you're have 100 Bitcoins when you monthly income or wealth aren't even close to 100 Bitcoins, this will raise suspicion into your coins.
Example you've explain you bought the coins at x price and your coins rose to y price, then you make z gains. They will not automatically listen to you, they will ask many many proof and even accuse you for doing something illegal which make them have a right to seize your coins.