Back from the vet's! And big sigh of relief! My vet took a look at it and said it's harmless. Something like a wart (not exactly a wart, but something similar to it, and BENIGN!!). She suggested not to remove it, unless it's growing (which could happen, but not neccessarily has to).

Anyway, she said it's something that's very common in older cats.

Thank you very much for your support!

Kirsten