Legitimate question:

Instapundit.com

"If sticking a device on someone's car -- which is a trespass to chattels in tort -- is okay without a warrant, then is it okay if I stick a device on a police car so I can track where the cops are going, and index it against a list of known donut shops? Or would they find something to charge me with? And if the latter, then why doesn't the creativity go both ways?"