My impression of the UA shell is that it is a quarantine zone or no-fly zone or DMZ of sorts. Not for us, of course, but rather between Peter and Paul. For the sake of argument, let's say the UAs belong to Peter, and the UPs belong to Paul. If the UAs scan ships and transmit their results, then perhaps they're specifically watching for Paul's ships.
Now I see in the
UP Tests doc that Test 28, "Honk at a UP with a UA nearby" may have produced a change in audio. Now I believe folks have tested to see whether the UA will scan a UP, with no obvious result. But I wonder if perhaps the audio transmission might be different between a UA scanning your ship with and without a UP in the hold. In the context of my proposed scenario, the UAs might be watching to see if the UPs are being smuggled out of the quarantine zone.
In any case, as this is a testable hypothesis, it might be worth adding to the list of proposed tests. Or you could, you know, try honking at the UP at a barnacle site, dunno why nobody has done that one yet--seems obvious, amirite?