Small Transport; Low Wing; 2 Turboprop Eng
Air Traffic Control Fully Certified
Air Traffic Control Time Certified In Pos 1 (yrs) 6
Confusion
ATC Issue All Types; Conflict Airborne Conflict; Deviation / Discrepancy - Procedural Published Material / Policy
Human Factors
I was working combined Radar Approach Control from the east side R3 position.  Aircraft X was descending west to east on an IFR flight plan to UES.  I received Aircraft X from ZAU descending to 110 and continued his decent to 050 to comply with MKE LOA.  I received Aircraft Y from MKE Approach Ripon sector north east of UNU airport level at 060 enroute to SUS.  Somewhere between UNU and RYV I inadvertently terminated control on Aircraft Y resulting in a dropped data block.  The target still showed a splat with altitude.  As I was scanning Aircraft X's descent to see if he was going to make his descent per our LOA to 050 before entering MKE airspace.  I saw the 060 altitude splat over RYV without a data tag and assumed it was a VFR aircraft.  I issued traffic to Aircraft X told him to stop descent.  Aircraft X leveled at 063.  I then realized that this was not a VFR aircraft but Aircraft Y enroute to SUS.  Before I issued traffic to Aircraft Y he reported aircraft in sight as the CA alert chimed.  Aircraft Y passed behind Aircraft X at an estimated 2 miles and 300 feet vertical.  The event happened due to my inadvertent dropping of the IFR tag enroute to SUS; and failing to realize that the tag was an aircraft I had on frequency and was IFR.  My scan missed the dropped data block.To prevent this from happening I need to pay particular attention to tags I drop.  I also need to slow my scan to ensure I am aware of all targets on my scope.
MSN Controller drops IFR tag not realizing it and later has conflict with other IFR aircraft causing a loss of separation.