Have you actually seen this happen while you were there or is this just something that has been related to you by the HO?
VERY strange that the problem persists even though you changed the offending detector several times. It would be IMPOSSIBLE for all to be defective especially when you substituted a smoke only for the combo. I would also say that it would not be possible for another detector or a wiring problem to cause this if all test normally when you press the test buttons and they all have been wired according to the BRK instructions. Just to be sure give BRK tech support a call at 1-800-323-9005 and ask if any of this is possible.
So, before you go chasing your tail with this (with only the HO's "diagnosis" to go on) and start replacing wiring or detectors, consider first if the HO is actually correct. Something is probably going on, but not bad smokes.
I'm willing to bet that that smoke is going into alarm for whatever reason and the others are also (as normal) and the HO just isn't picking up on it. That high frequency squeal can be very misleading as to where it is coming from.
-Hal
