The part about "that we as pilots don't hear" can be thought of two different ways - one being what was mentioned already, and the other being that while you're tuned to one frequency, I'm usually monitoring 7 when I'm working the Tower position (we call it Local Control), and when I key up, I transmit on 6 of those. We have a frequency called DECF that is used for emergency aircraft that are able to switch to it, as it is also monitored by our ground operations and emergency/ARFF vehicles. Otherwise, the other six are three UHF and three VHF frequencies, and depending on which runway you're using, you could be on any of those three. You may hear me talk to someone but never hear a response; that's why-- they're on a different frequency.
As spades mentioned, we have a system called TEDS that lets us key up all of our facility's frequencies, and occasionally, we'll turn on the emergency backup transceivers, mainly to test them as part of a weekly equipment checklist, but we also occasionally tune in a satellite airport, particularly if we had a Class B airspace violator. We know they'll key up to announce their presence to the satellite airport even if they didn't talk to us.
As for the guard frequency (121.5/243.0), we just have it monitored on loudspeaker at a station in the back of the tower cab. It's amazing the number of pilots that don't realize they're transmitting on guard.
I added the UHF frequencies to my STL feed so you could hear the Boeing fighter jets that depart (their callsigns are preceded with "MAC"), and they're also the frequencies used by military aircraft in general.