Yes - they look like civil registered privately owned relatively unmarked jets.
they are painted all white with a red and blue pinstripe down the side with a small NASA logo on the tail not likely any larger than 15 inches across.
here is a photo of their typical paint scheme - this particular F-5 has had its nose - belly and cockpit area very slightly modified for supersonic testing - it looks very much like it would be privately owned. ie. no stars and bars, little or no official markings of any kind etc.
(low quality photo)
http://www.rol.ru/pictures/misc/spacenews/f-5e.jpgi have not been to ellington field in houston texas in quite a while but the last time i was there these guys were landing 1 and 2 at a time about every 15 minutes - same story - unmarked white F-5 tigers - only way i knew they were nasa was that i was holding short of the landing runway and every time they would roll by i could see NASA on the tail. they also had a Cessna Citation with the same markings. i asked about the aircraft and one person told me that the citation is modified to look like the shuttle cockpit inside and they do power off glides in the Citation from high altitude into ellington and other large airfields all along the gulf coast and into the interior of texas and florida.
i cant say with any certainty that what you saw was a NASA F-5 tiger 100% because i didnt see it with my own eyes, but if one went over your head on approach to landing the paint scheme would certainly look very civilian in nature.
what i can tell you with a good degree of certainty is that NASA F-5 tigers routinely operate into new orleans, houston, austin, san antonio, tyndall AFB (?) and various large airports in florida, and that anyone who captured a fleeting glimpse of one of these guys would almost certainly insist that they were privately owned.
you could do an N# search on the internet if you know the aircraft N# but short of going to an FBO and simply asking "who runs the F-5 tigers into and out of this place?" those would be the only sure fire ways to find out for certain.