Right, @Tess. You've only got one option remaining to you at this point: scary, threatening, TV-show-esque INTERROGATION.
Line up 3 chairs in the kitchen, or if you have one, a big, mostly empty garage. The emptier, darker, colder, more cavernous, and more echo-ey the space, the better. Turn off ALL of the lights in this room, and in the surrounding rooms. Do this at night for maximum ambient darkness.
Arrange the chairs in a row, but leave room for yourself to pace before and behind them, and room to walk between them. Arrange them so that the person sitting in the chair feels sufficiently isolated.
Gather (or acquire) 3 very harsh lights, whether torches, bendy-neck lamps, or those harsh, clippy work lights builders use. They can all be identical, but if all the lights are different, and of different brightness, that's great; use the harshest on your chief suspect OR on the suspect you believe would cave first if they knew anything. Place some sort of table or stands a metre or so in front of the chairs, leaving yourself room to pace before and behind the lights as well as around the chairs. Place the lights on the table or the stands, and aim the lights directly at the chairs - one light, one chair. If the table/stands are loose and wobble with each of your footsteps, good. Use that to your advantage.
Place a small recording device on the table or stand.
Find yourself a smart, but intimidating business suit - preferably skirt, but trousers will do fine. Featureless, one solid colour, or the closest thing you can find. Plain white shirt under the blazer. Minimal jewelry. Wear simple, but heeled, shoes with loud, clicky heels, so that every step you take echoes through the room. Pull your hair back away from your face. Find a thick stick, large wooden spoon, riding crop(!), or some other intimidating looking prop to carry in one hand; carry nothing else.
Gather the 3 suspects into the darkened room by mysteriously calling out to them from inside, in the dark. Stand behind the glaring lights as they enter, so that they're forced to look into the lights but still cannot see you. In an authoritative, emotionless tone, instruct them to sit them in the chairs. As soon as the first sits down, for no reason but to confuse them, say "No, you, sit there, you sit there" pointing with your prop, from behind the lights still, so that they can only see the prop. Then make a point of visibly turning on the recording device.
As you emerge from behind the lights and walk to the centre of the space before the 3 of them, explain to them, in that same emtionless tone, why you've gathered them there: "There has been suspicious activity in this house recently, and we are gathered here to get to the bottom of it, right now. Nobody is leaving this room until the matter is settled." Meticulously go over the series of events, pacing and waving your prop, occasionally pointing it in the faces of the accused as you refer to them.
As you question them, pace and pace, clicking those heels. Alternate between pacing (ever so slowly) in front of and behind the lights, in front of and behind the chairs, as you ask general questions of all the suspects. Alternate between bending down into their faces, and standing tall with arms crossed directly before their chair, as you ask pointed questions of each individual. Occasionally tap your prop into the palm of your empty hand, making a smacking sound, to emphasise certain words as you speak.
Pit them against each other. Use the old but reliable "if one of you doesn't confess, all of you suffer" line on them. Turn their answers around on them. Catch them in any lies or discrepancies. Drag the truth from them forcibly. Maintain that emotionless, authoritative tone throughout the interrogation.
Get to the bottom of it.
Noooowwwww, unfortunately, I just can't seem to come up with the questions themselves, or any sort of speech or opening statement! I can set the scene, but I need some other creative soul to help with a script of questions to ask, intimidating verbal jabs, jukes, ways to get under their skin, and make them believe you know more than you actually do.
Surely there's some lovely, creative soul up late and willing to help me build my fantasy @Tess get to the bottom of this!!??