That she knew of its existence made sense considering her own history with the Beckwith house, the question now was: why did Miss Camden go in the garden through the hidden door instead of the gate beside the front of the house? Was her attention diverted? Doing that meant that she had to have passed the study on her way even if entering it wasn't her first intention, did she witness the murder? Did she hear or see part of it? Why would she decide to conceal this segment of her story? Maybe he should seriously consider starting to suspect her involvement. Yet, his instincts wanted to convince him this couldn't be it, she couldn't be the murderer. He had met many people who had killed a person: they were either carrying debilitating mental scars, or had a coldness inside of them that in one way or another showed through their lies. The detective just couldn't see it anywhere, even imagining her to be the one: her collected countenance was only cold to the touch, but hot on the inside, burning of repressed anger paired with grief, of pride and silent defiance. Or maybe he was, once again, overanalyzing a common woman, letting her cloud his judgement.

Then there was the mysterious man – for a man it had to be, no woman present at the party had enough height to pull off the action - who had put a wine bottle filled at least halfway with some strong alcohol on the counter so that it could get served to the guests. Alderton did a mental list of all the men who could know about the secret passage: Mr Beckwith, who was one of the more sober people at the party – his absence had no way to be inconspicuous, and he could just walk into the kitchen without raising suspicion if he wanted to; Mr. Hastings was very busy throughout the whole night, and he showed too much genuine familial love for the Camdens to not feel guilty about participating in the brother's demise, if he was good enough of a liar to fool the detective then he would humbly accept the consequences of it; his son, the cook, was as occupied as the father, and no one would have faulted him for being in the kitchen or even carrying a bottle of wine; the gardener who allegedly went to visit his family after finishing his work for the day, about whom he knew nothing, and who could have very well have stayed within the premises with the invisibility bonus of no one looking for him as they believed he wasn't there; Mr. Camden, before dying, and no one mentioned his additional sortie to the one he had done later with Mr. Davenport. 

There was one obvious first choice in the detective's mind.