Green Bank Observatory does not intentionally transmit, and any leakage would hardly exist for long, since the whole point is to listen to extremely weak signal

Green Bank Observatory does not intentionally transmit, and any leakage would hardly exist for long, since the whole point is to listen to extremely weak signals. However, a radio telescope differs from your usual radio receiver in that the stages are often separated by vast distances. I'm unfamiliar with the exact signal chain used in GBT, but I do have plenty of experience with a similar scale telescope, the Arecibo Radio Observatory. A radio telescope will go to huge lengths to reduce the system noise of the receiving elements, including cryogenically cooling the receiver and first few stages of amplification. These signals will then often be brought down within a shielded enclosure (often within a shielded receiver room) and shifted into their intermediate frequency (IF) stage with a highly pure local oscillator (LO) signal and brought back down to the control/electronics building. In the case of both GBT and Arecibo, that IF is brought down as modulated light over fiber optics, meaning that there is no RFI leakage possible on the transport. And at both, the conversion back to IF is done within a shielded room. At GBT, if I remember correctly, that room is within the Jansky Lab, which was designed as a shielded building. All of this is to say that it's very unlikely that there is any signal at all transmitting out of the telescope system in any way.