Although this is off-topic since it is about economics I'm going to write a short answer in case it is decided that it is on topic. I will remove this if it gets migrated.
The key to why printing extra money causes inflation and a change in exchange rate has nothing at all to do with whether or not it is announced or done in secret. The quantity of money in circulation (by whichever measure) is what informs the value of the currency not whether you tell people that it has changed or not. Announcing it should increase the volatility somewhat around the time of the announcement but the long term effects will be the same no matter what.
Whether or not they announce it printing more money will mean that there is more in circulation which will reduce the value of each unit of the currency by the dilution amount which in turn will increase inflation and decrease the exchange rate.