It is really upsetting to think that people close to you may not be loyal, and if this object is indeed yours, then what person A did was stealing! Not a pretty label, but accurate. You are feeling guilty for suspecting this, but your instinct is telling you that this is what probably happened.
It is a tough position to be in, but if person A has nothing to hide, they should have no problem showing you the object in question. I realise that your issue is how to raise this topic without implying theft (just in case you are wrong), and also maybe because you are not supposed to know about it....
You have a few options - try to get B to show you the object before it is returned to A, call A and just chat and catch up, casually mention that you have heard that they have such-and-such and object, and pretend nostalgia and just ask if you can see it (memories and all that).
Or if you don't value any relationship with A, and the communication between A and B was public (you are not breaching a confidence), then an outright request to see the object as you feel it might be familiar would not cause you to lose any friends...My feeling is that if they are innocent, they would be upset but eager to show you the object to prove you are wrong, if they are guilty they would also be angry (at being caught!), but would use 'righteous' indignation as an excuse to not show you the object as they would know you would recognise it - they would turn your guilt at asking back on you 'how can you think that" etc, but they would refuse you access to the object and say that you have no right to ask....
It is a really difficult position to be in and I am sorry you are experiencing th emotional distress of it - I hope it gets resolved soon.
Best of luck!