I did some research,and found this aircraft (the Citation) is msn 750-0054 and was registered N45ST,but became N450T,and was still registered thus in 2004.
However,this too is no longer current(latest allocation was to a Cessna P210).
So you see how difficult it is to trace US registered aircraft. You really have to get a manufacturers serial number at time of sighting,so that you can follow it's history through it's life.
I'm off to find it's current identity! Wish me luck.