Am I liable for anything for having used a program on my laptop with a fake serial number that the maintenance technician put on it without my knowledge, and which I have used throughout the past months, thinking it was free?

If a software company prohibits the use of its products except for a fee, then it is not permissible to infringe upon its right, except in exceptional circumstances. And if a maintenance worker installs a non-free program without your permission, then the sin is upon him, not upon you. However, you must delete the program from your device once you become aware of the religious ruling.