Law Enforcement Agencies have started buying new, portable radar devices that allow them to detect movement through walls. Essentially, they can tell how many people are in a house without going inside. They were initially developed for the military, but are spreading through law enforcement agencies across the country.
This is inherently problematic, because there is an expectation of privacy in one’s home. The Supreme Court has previously made it clear that such an invasion of the sanctity of one’s home requires a warrant, or is otherwise unreasonable. USA Today correctly covers the law on this: while the radars are new, the Court has previously and unequivocally declared that similar means of looking inside homes is not Constitutional without a warrant. The court issued a ruling several years ago about using infared scanners on homes, and more recently prohibiting dog sniffs of homes without warrant.
This can be an effective tool for law enforcement, provided they do it right. They need to get a warrant before utilizing radars and other devices that provide information from inside the home, and they need to make explicit in their application that they wish to use the device, and why there is probable cause to support it: otherwise any evidence obtained from such as search will be inadmissible.
via USA Today