Unfortunately, Authorize.net messages are not clear. When you stored customers' credit card details , Authorize.net can contact the card issuing bank to validate the card.
This, however, creates an "AUTHORIZATION" transaction of 0.01$ visible in the customer's card statement. It is not a charge, but a temporary "hold" of the funds, for 1 cent.
By default, Bookeo does not use this mode, as it creates unnecessary confusion. Many customers would think that they were charged 1 cent.
So when storing credit card details, Bookeo tells Authorize.net to not do this extra check. And Authorize.net calls this "TESTMODE", which has nothing to do with your Authorize.net account being in "test mode".
If you would like to enable validation of card details with the issuing bank when collecting card details (and understand that customers will see a 0.01$ transaction in their statement), please contact our Technical Support Team at support@bookeo.com and we can enable this for you.