The till receipt should be traceable through the back office,you say they were two other transactions,every time a transaction is done at a till the items sales records it ie which till, what time, what operator, it will also tel if it was key entered, or swiped,who ever did it would not hang around and would have gone to exit, provided it wasn't staff and the exit /entry cameras are better, some stores have the digital systems, which only take so many frames a second,so it might look like it just disappeared,but you should get a picture off another camera of someone hanging around or exiting,