Radar

The Radar is an item that detects nearby enemies. It is one of the three parts necessary to make the R.E.K. 3000, with the others being the Lifeform Analyzer, and the Tally Counter. If you are within approximately 20 blocks of another player on your team, who has the Radar, you can also see the enemies detected by their Radar. It can be sometimes found in wooden chests.