Randomly determine whether an AI is "sentient" or not; the builder doesn't know, he just uses his AI every turn to build things, from his point of view it gives him random bonuses, but sometimes a new player gets added who has takes the decisions, and gets some extra actions on the side too, which his owner may or may not notice (he may choose to reveal himself).

This also solves the problem if a player wants to build an AI, but there is no new player willing to join the game at the moment.

Actually, the game should make it difficult to find out whether the "AI" is really an AI or a human. For example, there should be a few different AI scripts, so unusual human behavior seems like another script. The AI script would sometimes, but very rarely, make a random stupid move, to provide plausible deniability to human action; however the damage should be relatively low, so the AI bonuses make it on average a net benefit to have an AI.

On the other hand, even if there is a human player, there would be a script assigned and it would suggest default moves, allowing human to override any (possibly even all) of them. This would allow the human to seem more like a script; mostly letting the script do its work, sometimes override their moves to gain strategic advantage. Or take full control, if they believe it will not be suspicious.

Also, the AI would not have to get "sentience" at the very beginning. For example each turn there would be a 20% chance that the game will open the AI to be taken over by any new human player, so you would never know when exactly it happened.