Directed communications (masers and whatnot) anywhere in a system should be allowed, and IMTU are how most conversations are held.
Because of that, most ships IMTU do not carry sophisticated wide-space scanning equipment, relying on MASER (or similar) directed energy systems for communications.
Here's how I break it down:
For transmitting signals:
All ships have direct beam communication gear that can direct a signal to any destination that the ship can lock onto. This beam's dispersal rate allows some "slop", and it's possible to "spam" a small angular cone if your broadcast system is down. This technology allows relatively private conversations between ships.
All ships also have a broadcasting (generally radio) system that sends out a signal in all directions. This is used for the ship's transponder but can also carry the GK (mayday) signal as well as voice communications if necessary. For safety reasons, this system often has a battery backup. (This system is equivalent to the old practice of "flying the flag")
For receiving signals, both man-made and natural:
Most commercial ships carry what would seem today like fairly simplistic "whole sky" scanners to detect the "broadcast" signals - GK, Transponder and Starport (or an associated beacon) homing signal. These systems are generally low-power active and passive RADAR systems that can do double-duty as signal demodulators.
Basic sensors are the minimum levels required by Imperial law, and can detect Starport signals anywhere in the system (especially since the ship will know about where to look for it), as well as GK and Transponder signals within a close enough range that a 1G ship could feasibly maneuver to give assistance. (Age of Sail equivalent would be the spyglass - just enough of a sensor to ID ships' nationalities and spot land.)
Civilian sensors for "local authority" type ships allow them to detect Transponders and GK signals at longer ranges, but aren't really sensitive or precise enough for monitoring the whole system. (Besides, with light-lag and travel time, a government that wants a presence in the outer system will need to put ships out there.)
Military grade sensors are more like what we'd think should be possible - long range and sophisticated analysis of any radiation, including "boring" civilian activities like Transponder tracking but also including scientific missions and very long range scanning.
Other MTU comms miscellany:
I don't consider the basic landing/docking RADAR equipment to be part of the sensor package, but rather part of the bridge electronics. As such, it's relevant to Piloting and not Sensors or Communications skills.
Transponder signals carry very little information (mostly the equivalent of a GUID and classification for the ship) and are used to help get a passive lock on a ship so that more detailed direct communications can be established. It's easy to turn on/off the transponder, but much more difficult (and illegal) to change its signal.
Imperial Law requires ships and boats to transmit transponder codes, though some captains in dangerous territories only broadcast a code once they've identified what other ships are in the vicinity. No ship is allowed to approach within 100d of the main starport or depart from a starport without activating its transponder, and having to be asked to turn on your transponder is likely to get you boarded or even attacked, depending on the situation.
The ship's computer remembers all transponders detected and keeps a database of known ships (auto-ship-spotting). Since the transponder signal is not very detailed (the ship's class is about all you can get from them), the database entries may be very empty. Players can theoretically "ping" any ships encountered to exchange news and such, but not all captains take kindly to nosy neighbors...
