I think that you mean the Secure Code of a Master Card and not a Maestro Card.This is the CVC code, that you can use for online shopping.
Only a Bunq Creditcard from MasterCard are provisioned with a Secure Code. A regular Maestro Card will not have a Secure Code.
With Bunq, this Secure Code is not printed on the card (from security perspective a very good think). The secure code from the Bunq Master Card you can find in the mobile app, and this secure code will change every x minutes/seconds so that when your secure code will be compromised, it will be only a very limited risk (time) that this code can be misused.
Let us know if you have any other questions.