Hello,
I don't know what aircraft this was but airplanes do have a TRANSMIT (TX) RECEIVE (RX) light - at least the aircraft I fly does.
When we press the PTT, the letters TX appear on all our screens along with the frequency we're transmitting on. When someone else speaks on the radio, the words RX appear.
I have come to the habit of checking that the words TX disappear every time I let go the PTT button to make sure I'm not transmitting anymore and avoiding unwanted open mic.