None of them are correct sentences because "device" has no article. You'd need to say "A/the device is (not) found". However, you will see "device found" or "device not found" pop up on a device. Neither of these are full sentences, but this form is commonly used in titles, announcements, signs and, more recently, computer pop-ups. Someone asked about this earlier today. You can see the question and the responses here: http://www.italki.com/question/316982

Those sentences are all written in what is called "passive voice", please read this article: http://www.englishpage.com/verbpage/activepassive.html The first two sentences need a "The" in the beginning, to be (grammatically) correct. They should say "The device is/is not found". Also, keep in mind that the above is grammatically correct, that doesn't mean it can be used in every situation. For example, if your friend Karen lost her keys, and both of you are looking for them, if you find them, you shouldn't say to her "The keys are found!". You would use active voice and say "I found the keys" or more casually, "(I) found them!" The only situation where the last two sentences you asked about sound natural to me is if, for example, you are trying to use a program to use your webcam on your computer. You turn the program on, and it tells you to click "Search" to find your webcam. You click "Search", it searches for a second and then finds your camera, saying "Device found". Basically those sentences sound a bit like robot/computer talk, because of the passive voice, and the use of the word "device". Hope that wasn't too complicated :).