Can't remember exactly where. It was an independent system. I was travelling from Imperial space to Federation, after I filled up with Imperial Shielding and I stopped to scan the NAV beacon...
Back to OP: I located an HGE in Shinrarta yesterday evening, but unfortunately it was PLA-PRA. No PIs for me. Anyway, I farmed it for a few times until I was full of PRAs.
It was like 3rd time when the HGE appears after I scanned the NAV beacon, while traveling in supercruise (after about 30 seconds of browsing on the navigation panel). Strange. It was a brand new generated HGE (40 minutes). This means that it's worth checking more than a simple scroll the navigation panel after entering supercruise. The HGE was not USS, but HGE from the start. So it seems that after scanning the NAV beacon, it takes a while until every USS is identified or shown in the panel. Or maybe the new generated ones appear there if the scan was just a short time ago, I don't know.