A.T. Robertson Commentary Mark 14:66

"And as Peter was beneath in the court, there cometh one of the maids of the high priest;" — Mark 14:66 (ASV)

Beneath in the court (κατω εν τη αυλη). This implies that Jesus was upstairs when the Sanhedrin met.Matthew 22:69 has it

without in the court (εξω εν τη αυλη). Both are true. The open court was outside of the rooms and also below.