Theory: Not just the military will want alien probes. Some organisations will also want to obtain them: for experimentation, sale, or leverage. Such organisations will need a huge amount of resources, both in terms of weaponry and finance.
Therefore, I shall now concentrate on looking in systems under the control of a corporation. In addition, since there will be some skirmishes involved in their protection, transport and other logistics, I shall focus on signal sources suggesting high-threat weapons fire. The systems best suited to this would be anarchy.... mainly because EDDB has roughly 1481 "*corp*" wildcard systems, but only one anarchy.
... but there's only one anarchy "corporate" system: Robigo. Even though they're a corporation, the system itself is labelled as Anarchy, which I find odd. Everyone always knew that this was the most crooked corporation in the game anyway. So, I shall be hunting there. I'll also be dropping in on convoys as well.
I can also note are also these Corporate systems that are in a Civil War state (according to EDDB) and might be more likely to spawn "weapons hot" style USSes.
Quikindji
LO Muscae
Hardh
Namab
San Guan
HR 3007
Paribela
Yen Hsini
Col 285 Sector KS-T d3-78
It might also be worth trying to find a system, maybe Corporation, that does *not* have Unknown Probe listed as a prohibited item.
On 5/6/7 Aug, I shall be searching these systems:
YZ Canis Minoris
LHS 1963