G4 CREW SAW A NEAR COLLISION BTWN AZ AZTEC AND A CESSNA AT AFTER LNDG AT UGN. THE TWR WAS CLOSED AT THE TIME.

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ACFT Z -- CESSNA. THE TWR WAS CLOSED AT THE TIME OF THE EVENT. AT APPROX XA06 ON JUL/XA/01; ACFT #1 LANDED FROM AN ILS APCH ON RWY 23 AT UGN. IN A VISUAL PATTERN (L DOWNWIND) AT THE SAME TIME WAS AN AZTEC (ACFT #2). BOTH ACFT SAW EACH OTHER AND ACKNOWLEDGED EACH OTHER'S POS ON CTAF. THE AZTEC FURTHER ACKNOWLEDGED OUR POS ON CTAF. THE AZTEC FURTHER ACKNOWLEDGED OUR CLRING RWY 23 CALL. AFTER LNDG; WE CALLED ORD APCH ON FREQ 120.55 AND GAVE THEM OUR DOWN TIME BECAUSE WE HADN'T YET CANCELED IFR. WE CLRED RWY 23 AT TXWY A2 AND TAXIED TO THE INTXN OF RWY 5/23 AND HELD AT TXWY B TO WAIT FOR THE AZTEC TO LAND. WE SAW THE AZTEC ILLUMINATE HIS LNDG LIGHT AT APPROX 300-400 FT AGL ON SHORT FINAL. A FEW SECONDS LATER WE SAW HIM MAKE A HARD; CLBING L TURN; GOING AROUND. SIMULTANEOUSLY; WE NOTICED A CESSNA (ACFT #3) AT APPROX 200 FT AGL THAT HAD JUST TAKEN OFF FROM RWY 5; UNANNOUNCED. WHEN WE SAW HIM HE WAS ABOUT OVERHEAD THE INTXN OF RWY 5/23 AND TXWY B. AT NO TIME DID WE HEAR THE CESSNA MAKE ANY TA CALLS OR ANNOUNCE HIS POS/INTENTIONS. THIS IS WHAT CAUSED THE PROB. CONTRIBUTING FACTORS INCLUDED NIGHTTIME; VISIBILITY LIMITED DUE TO FOG AND MIST; AND A CLOSED TWR. ONE SOLUTION WOULD BE LONGER HRS OF OP FOR THE TWR.