I use this code on a loop with InTheHand.Net.Personal.dll (v 2.2.0) on windows XP 32bit:

BluetoothDeviceInfo btdev = new BluetoothDeviceInfo(btAddress);
btdev.Refresh();
Console.WriteLine(btdev.LastUsed.ToLocalTime().ToString());

Console.WriteLine(btdev.LastSeen.ToLocalTime().ToString()); 

the LastUsed always is equal with LastSeen

but the device comunicate with computer (it is a HID device, like a keyboard, the actions can be monitored easy) and if i use a DiscoverDevices,  LastUsed is reported well.

what to do not tu use  DiscoverDevices ?

10x

Those properties are read from the Microsoft Bluetooth stack in this case.  Unfortunately they're often wrong. :-(  I keep meaning to ask MSFT if they're planning to fix this anytime

In detail, I thought that LastUsed was always wrong, always just containing the current time when the discovery operation was carried out -- even for devices not in range!  I thought LastSeen was kind of correct, containing DateTime.MinValue mostly but if the device has been used in the current boot then it did contain a correct value.  That's what it seems like on my XP box here.