The radiation from Tc-99m is mostly 140 keV gamma rays; the radiation from Tl-201 is mostly 167 keV gamma rays. The sensitivity of the border inspection detector would be roughly the same for 140 and 167 keV gamma rays, and I would expect the physician to inject roughly the same amount of radioactivity for each type of test.
After four days, 16 half-lives of the 6-hour Tc-99m, its activity would be down by a factor of 64,000. Tl-201 seems to be the more likely possibility.
Maybe all these radiation measurements will help introduce a sense of realism into our social consciousness. Did you see Ted Rockwell's article in the Dec 2004 issue of Nuclear News? It's about the new ANS project, "The Realism Project: It's time to get real."