COS TRACON Controller described unauthorized entry into adjacent airspace; claiming stored data information on the military flight as well as handoff symbology caused an assumption that a handoff had been accomplished.

Narrative

I accepted a manual handoff on a flight of 2 military jets from D01 (Denver Approach) publication (Pueblo Sector) returning to BKF. I was somewhat busy with numerous other aircraft and a persistent stuck mike. The coordination of the transferring Controller indicated that the flight was IFR though the aircraft was squawking a D01 local VFR code. The tab list included a call sign; so I initiated control of the data tag by slewing to the target. I could have assigned the correct code to the jets; but his transponder had been intermittent and I knew it would be faster to tag him up using the slew. I then acknowledged the flashing beacon code indicating a discrepancy between assigned and expected codes. Because the aircraft was still on the D01 VFR code; D01 South Departure sector did not have a data block on the aircraft. As the flight approached the airspace boundary; I observed that the data block was on a 'C' position symbol and I assumed that it had handed off to D01. The aircraft entered D01 airspace without handoff or coordination.