If I have a unit at a battlefield after holding and scoring and move it back, if I move another unit in does a showdown start even though it has been scored already off the hold and there are no other units there?

Yes, a showdown does start.

According to RiftJudge FAQ #7022 and FAQ #6028, a showdown occurs whenever a battlefield becomes contested, regardless of whether it has already been scored that turn.

Here is the breakdown of why this happens:

1. Losing Control: When you move your last unit out of the battlefield to your base, the battlefield becomes unoccupied and uncontrolled (Rule 323.6).
2. Contesting: When you move another unit back into that battlefield, it becomes contested because you are moving a unit into a location that you do not currently control (Rule 445).
3. Showdown Trigger: Because the battlefield is contested and the turn is in a neutral open state, a showdown begins (Rule 344).

Important Nuance: While a showdown is triggered, you will not score another hold point or gain another conquer proc. The restriction on scoring applies because the battlefield was already scored earlier in the turn (Rule 465, FAQ #7447).

Additionally, because the battlefield was unoccupied when the showdown began, this will be a non-combat showdown (unless your opponent also moves a unit there simultaneously, creating a combat scenario). Per Rule 348.2.a, since you are the only player with units present, you will regain control of the battlefield once the showdown closes.