In the hummering test and acoustic injection before/after the O3GK, we found the bellows between IFI and IMC were most effective to DARM but chambers and their legs were not effective.
So I guess the most serious ghost beam was coming back from the down stream to these bellows directly, not hitted any object inside the IFI chamber.

In the picture took from IMM chamber side today, we can see the aluminium hoil directly in the marked space.

Other pictures I took are here.