His outpost was obviously in scanning range of Outpost 8, the Enterprise was not.
Or then he had access to secure internal communications across the fortress chain, or parts of it, including enough telemetry that total destruction could be ascertained. He would have been blind (due to stealth measures on both sides) but not deaf.
Outposts 5, 6 and 7 might have been destroyed as well, but without enough telemetry to tell why they went silent. Or then silence from them was not unexpected at that point, but rather the tactical norm, and Hansen thus had no proof of destruction. Or then the Romulans tried out a "lone" outpost first before risking taking down three in a row. It's even possible that the loss of Outpost 8 was an error in Hansen's interpreting the communications and lack thereof; such things often happen.
In contrast to the other outposts, Hansen must have had time and/or extra energy, after his outpost's deflector shields had been obliterated, to send a distress call to the Enterprise and even have some extra time to wait for them coming into visual communications range.
Or then all the outposts sent out similar messages, but only a very proximal starship could pick them up from amidst the local (Romulan-generated or natural) subspace noise. And Kirk just wasn't close enough when the first two or three outposts were lost.
But personally, I'm ready to agree the Romulans just ran out of luck - either their weapon finally failed to perform up to specs, or Starfleet reaction time got better, or some other conditions initially deemed favorable for the attack eventually deteriorated.