It depends on the situation and the amount of people involved.

Consider a situation where a single researcher has managed to develop AGI in 2010, and to not have anybody find out about their research. They've destroyed their computer and their research, and moved to Nigeria to work on completely unrelated stuff. In that situation, I don't think it would be hard for them to keep their discovery a secret indefinitely. This might as well have already happened, as someone who figures out the path to AGI doesn't need to implement the AGI or to write anything down. They can keep it a secret the same way they can keep a secret on the time they thought to steal some wine bottles from Walmart but decided not to act on it.

Meanwhile, if a larger group had developed and implemented AGI (say a project involving 100 people working directly or indirectly on it), it would be hard for them to keep a secret. Believing that they could do that would pretty much equal the definition of a conspiracy theory. There are too many points of failure, and the incentive for them to use the AGI for some sort of (hard to hide) personal or institutional gain is too strong.