Receiving loopback signal without loopback cables or antennae (ADRV9361 on ADRV1CRR-FMC)

I'm trying to get started with this radio.  Here's a picture of the device:

I wanted to use Matlab to communicate with the radio, so I followed Mathworks' guided-host radio hardware setup instructions found here:

https://www.mathworks.com/help/supportpkg/xilinxzynqbasedradio/ug/guided-host-radio-hardware-setup.html

Then I ran Mathworks' test script which is intended to send and receive a test tone.  The script is here:

https://www.mathworks.com/help/supportpkg/xilinxzynqbasedradio/examples/receive-tone-signal-using-analog-devices-ad9361-ad9364.html

However, I did not attach a loopback cable between the TX1A and RX1A ports as instructed nor did I attach an antenna to those ports.  Even so, after running the script I saw the following result:

Which is exactly what the test script's website says I should see, but for the fact that I didn't have a loopback cable or antennae attached when I ran the script.  In an attempt to troubleshoot the problem, I changed the test tone to 50 kHz and reran the script.  I got the following result:

Which is exactly what I would expect to see (a peak at 50 kHz) if I actually had a loopback cable attached.  Next i tried putting 50 ohm terminators on the TX1A and RX1A ports and reran the script with a test tone of 25 kHz.  I got the following result:

The noise floor is slightly higher, but the peak at 25 kHz is still clearly visible.  Next, I commented out the line that tells the transmitter to start transmitting.  I got the result you would expect, which is that the 25 kHz peak vanished.  (Don't have a screenshot of this one, sorry.)

Am I misunderstanding the hardware or the software somehow, such that a loopback cable is actually not needed?  If this behavior is unexpected, what further steps can I take to track down the problem and restore the expected behavior?