SDL Controller described event when VFR aircraft; initially with Approach Control; entered SDL Class D without any type of required clearance; pilot indicating they were infact IFR; reporter indicating a communication failure between ATC and the pilot.

Narrative

While working local control at SDL i observed a C172; on radar; on a Biltmore sector VFR tag about 20 miles west of SDL. The scratchpad indicated the aircraft was destined for SDL. The Controller working Biltmore sector (TRACON) called and asked how weather looked to the west and north to attempt to get this inbound aircraft below the cloud layer; I gave my best estimate. I assumed the aircaft was going to request special VFR when/if he got below the clouds. About 10 minutes after the conversation with TRACON; I observed the VFR data tag enter SDL airspace from the northwest; without contacting the tower. The aircraft entered a right downwind for Runway 21 and only contacted the tower when he was abeam the numbers. I informed the aircraft that 'the airport is below basic VFR and to say intentions'; the aircaft said; 'I intend to land.' I exchanged several transmitions with this VFR aircraft expecting him to request special VFR; and he never did. In his last transmition the pilot said; 'I am IFR.' Not quite sure how to handle this; as his data tag said he was VFR; and that by this time the aircraft was turning base; I gave him the weather and cleared him to land. When the pilot landed we asked him to call the Tower. The Controller-in-Charge spoke with the pilot who again indicated he was on an IFR flight plan. I still don't know; if he was really IFR or who changed him to VFR. Tracon indicated that he gave the pilot a frequency change in a timely manor so he could call SDL before entering the airspace. The tracon also indicated to his knowledge the aircraft was VFR and was instructed that radar services were terminated just before giving a frequency change. Looking back; it may have been a good idea to indicate to the Biltmore Controller that SDL was IFR; while he should have known this already; he may have said something to the pilot and other questions would have been asked at that time. Also; had the Cessna understood that 'radar services terminated frequency change approved' is not something told to IFR flight in poor weather; he could have questioned the instruction.