In general, your Hestan Cue cookware, Cue-Enaled cooktop, and mobile device (phone/tablet) should be within 5 feet (1.5m) of each other for an optimal (Bluetooth) connection while cooking. Your internet router should also be close enough such that your mobile device has "full" Wi-Fi internet bars.
Note: Depending on a number of variables, It's possible to maintain a Bluetooth connection over a hundred feet (30m+) away. This would explain why a "far away" Cue cookware unit is still detected even when it's in another room. To make Cue hardware completely undetectable, simply turn off the cookware or unplug the Cue cooktop.