The elevator is locked, and only residents (or authorized guests) can select floors.
The concierge has some level of control, but they might not always verify with the resident.
So,
If the concierge recognizes guests for parties without calling up, you might be able to return by acting like a guest for a different resident.
2 If someone who lives on the 8th floor enters the building and uses the elevator, you could discreetly join them.
If the terrace is a common area, residents may go there often, and you could time your arrival with one of them.
Additionally, I think the following might help:
Many high-rises have stairwells that allow movement down but not up. If you could access a higher floor and find an unlocked stairwell, you might be able to walk down to the 8th.
If the concierge didn’t verify with your friend last time, they might allow access again with a confident request.
Pretending you "left something at the terrace" might work if the concierge is lenient.
2 If someone who lives on the 8th floor enters the building and uses the elevator, you could discreetly join them.