Flaps
One other thought: MCAS is active with flaps up only. The pilots cycled the flaps after the first couple of MCAS trim downs for a reason. There was the perception that the trim downs started when flaps were up. So, they found out some kind of logic. If the system logic had been known to them (Flaps up, MCAS on - Flaps dn, MCAS off), i.e. if they had been informed or trained to understand the MCAS system, they most probably would have associated this logic with MCAS, then set flaps and return.