Police were called on Thursday to deal with the incident and a man was questioned before being released.
"Derby County Football Club can confirm that officers from the Derbyshire Constabulary were called to the Derby County Training Centre at around 11:20am on Thursday 10th January 2019 following reports of a man acting suspiciously outside the premises," Derby said in a statement on Friday.
"It has since been confirmed to Derby County that the individual concerned is an employee of the footballing staff at Leeds United Football Club.
"The club is now in discussion with Leeds United club officials in relation to this incident."
Derbyshire police confirmed they had attended the scene and encouraged Derby to take revenge by inflicting defeat on Marcelo Bielsa's league leaders.
"Officers have just attended the Training Ground for @dcfcofficial after a suspicious male was seen at the perimeter fence," Derbyshire Constabulary's response unit said on Twitter.
"Excellent searching conducted & male was located. All checks above board!
"Keeping the team safe to bring home a win against #LUFC on 11th! #SpyingIsCheating.”
Leeds are in pole position for a return to the Premier League after a 15-year absence, but have lost their last three games.
Frank Lampard's Derby occupy the final playoff place in sixth but can close to within three points of automatic promotion with a win at Pride Park.