Upload & Sell: Off
Great advice here that I can't really improve upon. I would like to add a couple things though.
1. If my employer were to give me a polygraph, I would either - A. refuse and risk losing my job or B. Quit on the spot. If innocent, that would be an insult to me.
"Questioning", "interviewing", etc by either my boss, another employee or private investigator would also insult me and would probably lead to me quitting. I'm sure there may be some legalities involved in this as well that could come back to haunt you. Leave this up to the police, that's what your tax money pays them to do. They work for you!
2. Don't set up hidden cameras for security monitoring. That's another insult. Put them in plain sight.
3. Suspect anyone, and everyone, that had access. I hate to say this, but not doing so will partly leave your guard down. Just don't go around advertising that you suspect Mr. X or Ms. Y. . It is true that "one person has to ruin it for everyone." Don't see much mention of the landlord. A lot of times, it's the person you least suspect.
I worked for a locksmith years ago as a teenager. It's amazing how easy your average, big box store, locks are to pick. I can pretty much promise you that I could pick the lock on your front door with nothing more than a pair electronics tweezers. I've actually kept the same pair for 15 years specifically for picking my own locks in case I lose a key. When changing locks, don't skimp out. Only buy reputable brands and don't buy those "sets" where you get 4 locks that use the same key. This is a big no-no. Each lock should have it's own, different, key. Consider using a "cage" as mentioned earlier, where gear has to be checked in and out and only you have the key. Or it's an electronic lock and everyone has their own code. Each time it's entered, it's logged. Does it have to have a key? Use the types of locks that require a special, cylindrical, key. Like those used on vending machines. Extremely difficult to pick.
Last bit of advice....layers. But not the type we have in our favorite photo editors. Layers as in multiple security layers. The more layers, the more time is required by a thief. Time is a thief's worst enemy.
Good luck and hope you catch the spineless bastard that did this. Also keep in mind that the two incidents may not actually be related. You must consider any, and every, option.