Sector 2520 was a region of space located near the Federation-Klingon border. Lieutenant Aquiel Uhnari was captured approaching this sector as she fled Relay Station 47 aboard a shuttlecraft in 2369. (TNG: "Aquiel")
This sector was only mentioned in dialogue.
Based on the sector's seemingly close proximity to Relay Station 47, it would seem that this sector is in, or is relatively near the Beta Quadrant.