Tracking system: We have vouchers for every time someone asks for money for work, money out is also recorded in an old planner that has been converted to track cash disbursements. Liquidations are also noted against the cash outs they were issued for (to match cash disbursement and voucher creation). Only I am allowed to exchange bills and coins as requested by other people. Replenishment of the cash box has to be signed off a check request made every week, I have to personally make sure that the proper amount has been remitted to me me before I can sign that check request.
Box Security: The ONLY key that I know of to the box I currently keep with my house keys; meaning they cannot use the cash box if I am absent and/or not at the office. The box is kept in my desk drawer during office hours and kept in a locked filing cabinet that only me and one of the people I suspect have the keys to after work hours. I stay really late (because I watch Proleague in the office), so I know that they could not have taken money after-hours. In the mornings, I typically arrive later than they do, but I find the box still in the filing cabinet properly secured whenever I do so. I share a room with one of the suspects, and we (me + the two other people) share two rooms that are secluded from the rest of the company premises.
Circumstantial Evidence (possible motive): One of them currently has a credit issue with a bank and has been avoiding their calls over the last two to three months. The other has constantly made comments on me being "rich" and being able to take the losses if and when the losses happen; this person also happens to be the person who verifies my cash counts every start of the week, and keeps on telling me not to report the losses.
Given this situation, I want to find a way to catch them in the act of trying to break open the cash box AND/OR tagging them with any money that they might have taken on them before they can get any more. The best I've been able to think of is to leave my phone recording sound inside the drawer with the box (so I can hear them opening the box), but if anybody else has better ideas, I'd like to hear them.
P.S. If I can't prove it was them doing this, I definitely will file a fifteen-day notice to the company because I am sickened by this situation. Both of them have been with the company longer than I have, and I don't want to be the guy who gets thrown with the blame just because they can "get away with it" because it's my responsibility.