[Elecraft] Receiving/transmitting into an empty antenna jack

[Barry]

Decided to listen on 6m today for the first time, ever.  Heard a bunch 
of signals, then tried to tune my multi-band dipole with the ATU.  
Seemed to tune, but was erratic, and even worked a few stations in the 
contest.

Then, noticed I was on ANT2, which is empty.  Switched to ANT1 and it 
worked much better ;.)

How was it I was able to hear and work anything on (an empty) ANT2?

Barry W2UP