Thallium-201 has a half-life of only 73 hours. More than 99% of it would have decayed in 15 days (5 half-lives). This would have made it useless after being stored in a closed warehouse for months/years. It certainly couldn't be detected (as thallium, anyway) by even the best CSIs. These short lived isotopes typically are made, rushed to hospitals and used within a few hours to ensure their potency.