The thievery of "assets" by a person who has been granted access to said assets along with the capacity to operate or manage them is classed as embezzlement. It most commonly takes place in a corporate environment. Embezzlement is not in any way limited to the theft of physical assets; it can also come in the form of alteration of files and databases to cover up any theft of finances, this is classed as accounting embezzlement. If a person is given access and authority to operate the assets of a client or company strictly for the use of the former's best interest but instead misuses said assets for personal gain and interest, then this shall also be classed as embezzlement.
Depending on the skill of the perpetrator, embezzlement can be extremely hard to track down or even notice, that is because skilled embezzlers cover up their tracks with frightening skill. They have a distinct advantage in that they often have access to documents, files, and databases along with the assets themselves; this puts all the pieces in their hands. In the case of the major multinational companies, catching the perpetrators can be even harder as the records and cash flows are so vast that if the embezzler makes use of the multiple avenues, he can become incredibly hard to catch. Even if patterns get noticed, suspicions alone cannot prove anything and actions cannot be taken without proof.