Solution
This is not the signal and it happens due to wrong setting of the measurement. When you use "LO Configuration = Share", two channels sharing the LO should be enabled. If Channel 0 is not enabled while you share LO for Channel 1, the measurement cannot be performed and you cannot detect any signal.
For the correct measurement, you need to set LO configuration = Independent to use own LO for each channel. In this case you can still see some spurious but you can measure signals. For USRP-RIO, there is no specification about s