You can check the Scan Time tab of a Windows computer's Lansweeper webpage to see which scanning method or scanning target type scanned the machine and when. This information is also stored in the tblAssets database table, i.e. in the fields LastActiveScan, LastIPScan... This won't tell you the exact scanning target responsible for adding the asset, but it will at least tell you which scanning target type is responsible. In Lansweeper 6.0, scanning targets can be adjusted in the Scanning\Scanning Targets section of the web console.