As said, you need to vet a range of sophistication above the most sophisticated example you actually encountered, to assume there are no others that you could reasonably detect with the techniques you could deploy. Always make sure to know you'd see anyone who is only one level better than the best you encountered, where the size of such a level should be estimated from the density you see in the distribution of attacks.

You are also good if you don't automate defense with the best detection you have, so that you prevent an attacker from automatically judging the quality of your detection capabilities with you then believing the attacker got stopped when he just deployed a technique you can no longer see.

I.e., make sure you don't alert an attacker that you can still see him when you are just barely still able to do so, as you would not want him to up his camouflage to the point where you won't see him anymore.