What comes in a paperback-size box, can be attached easily to the bottom of a car and warms a private investigator's heart? Answer: LAS Systems Inc.'s "mobile trac" and "mobile watch" tracking tools, which use navigation satellites to see where vehicles roam. The Temecula, Calif., company said sales have been brisk, especially in states such as Ohio where such surveillance is legal. Michael Mullins, a gumshoe at Unique Investigative Services in Genoa, Ohio says the tool is handy for tracking alleged drug dealers, cheating spouses and truck drivers who take too long to make a trip.

The $2,995 wireless device holds a credit card-sized computer made by ZF Linux Devices Inc. of Palo Alto, Calif. It can send signals back immediately to a P.I.'s laptop, which can then display a vehicle's street-by-street movements. A $1,295 version can play back a car's entire journey. "If you have someone who has been followed before and is crafty, " says Mr. Mullins, " he can look all day long in his rear-view mirror, and we're not there." Except, of course, they are.