When you delete a file or folder in BeInSync the item is sent to the Recycle Bin on the computer you deleted the item on. On your other computers the item will be sent to the special deleted items folder of the share in the tree view.
You may click the deleted Items folder in the tree view (each synced and shared folder has its own deleted items folder) to access the files and folders that were deleted.
The deleted items folder will indicate of number of files not viewed. Every time a file is deleted, the number of unviewed files will increase, the unviewed deleted items will appear in bold; this way you can easily identify the most recently deleted items.