I don't think you are missing anything current...
That said I think there was something once upon a time where there was a test based on getting hit with a weapon (and firing it). I think the silent rule you have is ignoring the basic firing it check but the missing if hit but not at least downed the alarm is raised interaction seems to have been changed.
I think the sentry/sneak attack rules have been one part fo the rules which have gone back and forth so many times with each reprint of the rules, it has been difficult to track (and in my experience have to carefully check (prior to core 2023 rules) which version of the rules an opponent was playing,
I think the ‘raising the alarm’ test for firing weapon was consistent through each rule set; you rolled a d6, added the weapon STR and if over 10 it raised the alarm. Now it is just using any ranged weapon (without the silent trait) it raises the alarm.
The oddity was you could shoot someone, not raise the alarm… and then hit and pin them but the sentry still wouldn’t spot you (or the fact they had been shot). This did change in the limited ‘Hive War’ rulebook where an extra line was added to the bit about ‘if engaged’ to also cover ‘if targeted by a ranged attack and becoming pinned/prone’, but this then disappeared again in the Ash Wastes Rulebook which followed. It was brought back in the Core Rulebook, and applies to being hit by any ranged attack.
So I think its just that new line in the core rulebook about Silent weapons triggering the alarm on a 4+ which is new, and which contradicts the rules for the ‘silent’ weapon trait which indicate they don’t raise the alarm at all.
But actually, have just realised is more confusing as now if any sentry is hit by any ranged attack, this automatically raises the alarm… so actually, the silent trait no longer does anything unless you ‘miss’ the sentry.
I think I’d prefer that on the whole:
- firing any ranged weapon WITHOUT the SILENT trait automatically raises the alarm.
- firing any ranged weapon WITH the SILENT trait did NOT automatically raise the alarm (and required no d6 roll test), but subsequently
- if a sentry hit by a ranged attack was pinned or prone / seriously injured / out of action by that attack whilst visible and in spotting range of another sentry, this automatically raised the alarm.
- if the sentry hit by the ranged attack was still pinned and prone / seriously injured and prone when it is the defenders turn to activate a model, then this automatically raises the alarm.
- but if the sentry is hit, is not visible to another sentry, and is taken out of action by the shot (or before the defender next activates a sentry) the alarm is not raised and play continues.