GPS Trackers and Recording Devices
Tracking hardware is no longer only spy fare. There are somewhat affordable GPS tracking devices that can be placed on or in a vehicle. Some may require monthly fees, just like a normal GPS. They can come in handy for tracking a stolen car or other property, but they can also be used to track an errant partner, say one who has told you that he or she was working late. In these days of easily accessible spy equipment and Google Maps, it isn't that hard to catch someone in a locational lie. Plus, most smartphones these days have integrated GPS capabilities that can, if set up to do so, track your every move, as well.
There are also long-running voice-activated digital recording devices available for capturing incriminating audio. They can be used for more innocuous purposes -- taking audio notes or conducting interviews -- but like the GPS trackers, they have obvious snooping applications. And they, too, aren't just relegated to specialty shops. You can pick these things up at major retailers like Best Buy and Amazon, and hide them in a car or in other personal belongings that will travel with your partner.
Hidden cameras are also always a possibility. They can be stashed away in or disguised as common everyday items, like alarm clocks, or the nanny-cams that come hidden in teddy bears. But placement of these might be a little more difficult unless you know exactly where an assignation is taking place.
And there are, of course, private investigation agencies that will do all this surveillance for you, but they cost a pretty penny. With all the do-it-yourself devices available, not to mention the camera and recording capabilities of so many phones these days, it might just be best to assume your every move is being documented and act accordingly.