The verb detect means to discover or observe the existence of something. If you walk into your house and detect the odor of burnt food, you can deduce from the evidence that you will be going out to dinner tonight!
The verb detect comes from the Latin word detegere, which literally means "to uncover" — or more figuratively, to discover. You may detect the presence of a new dog in your house if you note a water bowl, chew toys, and grooming tools. Of course, the large woofing beast that greeted you at the door with a wagging tail might be a big clue as well.