The first episode of Star Trek in which cloaking devices were used was "Balance of Terror," which came at the end of Trek's first season. In this episode, while the Romulan Bird of Prey was running cloaked, it had trouble tracking the Enterprise, but wasn't totally blind. (Making active sensor sweeps would defeat the whole purpose of the cloaking device, because the other ship could just home in on the source of your sensor emissions. This was probably why the Romulan Bird of Prey couldn't "see" the Enterprise very well. However, the fact that it could track the Enterprise at all meant that at least some "passive sensors" were able to receive and analyze emissions from outside the cloaking field.)