There was a Supreme Court decision that said thermal imaging of a person's home is a "search" and requires a warrant.
http://en.wikipedia.org/wiki/Kyllo_v._United_States
Before this decision they could do a thermal image of your home and if it gave off a certain heat signature, the police could use that a probably cause to get a warrant to search the home.
Now, they need a warrant just to do the thermal imaging.