Try before catching a thief to enlist the support of colleagues that you trust unconditionally. Together you will be able to implement a number of measures aimed at the eradication of theft.
Contact your organization's security or to the supervisor in order for them to care about the safety of personal property or property companies. Tell us about the thefts, but don't talk about who exactly you suspect. Usually, if resources permit, the user shall issue an order to establish an in-house surveillance. The most principled leaders can send to all, without exception, employees (including you) to check on the lie detector. Although studies show that the truthfulness of the "testimony" made by a polygraph, fluctuate around 60-70%. Some bosses hire a private detective or embedded in the collective pseudo-workers, who for a fee, find out who deals with theft in the workplace.
If available funding is not (especially if it's a budgetary institution), then you will have to rely heavily on their strength and likely the help of colleagues, to create a provocative situation. Their organization depends on what specifically was stolen. Do not compile it yourself: if material damage was caused directly by your institution, please coordinate your actions with the authorities.
Try not to carry large amounts. All cash always keep to themselves. For the thief cook (preferably with colleagues) a false wallet with marked bills or spattering him indelibly.
Put the wallet in a prominent place, look away or even leave the room for a while. Agree in advance with other employees so they did not let anyone out of the room. Or loudly say aloud, "I think I left my purse in the dining room (accounting, admissions, etc.)". Then leave the room. Return note that place where you left the "bait".
If the wallet is not there, then, if you have used paint, note the hands of the audience. If you mark bills, tell your boss just ordered you to urgently raise money for (purpose of collection create in advance). See what bills will pass by all present. The amount of the fee, as their goal should be important to a potential thief probably wouldn't have enough cash, and he passed marked bills.
To find out who is stealing money and personal items, you can do it without bait. Bring along a digital camera. Before or after work (most importantly, that no one else was around), connect the camera to the computer, which you have involved the least. Try to find a secluded place that the camera was not visible, but she could easily take pictures of the event. The process of catching the thief in this case may take several days. If necessary, change the location of the camera.
Gather evidence. Do not attempt to deal with a thief, as assault and blackmail is also a criminal offense. Try gently to talk to the offender, if necessary, offer financial assistance. If the theft continues, and the scheme of its making is similar to the previous one, refer to the owner or to the police.