Weird Break in at a unit

[Samantha Klein]

I bought a duplex on 7/11 of this year and inherited two tenants paying way below market rate. When I took over, I issued non-renewal notices and one side was pretty upset while the other has been fully cooperative. The upset side asked if they could leave at the end of July which I agreed to, we did the walkthrough last night and everything looked good so I gave them their deposit back in person and had them sign a receipt and a release of possession form indicating they would not re-enter the property. 

I changed the locks last night and went home. I come back today to start making a supply list and find the front door unlocked. I walk in and immediately notice something is wrong. Both ceiling fans and lights in the kitchen and living room were on and I know I shut them off when I left as well as locked the door behind me. I walk into the laundry room and find a bunch of broken glass which appears to be the light bulbs and little chandelier bulbs from the ceiling fans. They left one bulb each in them but smashed the rest. There is no other damage other than my electric screwdriver I left on the counter is gone ($15 tool). 

These property has old windows, the type you have to pull in tabs to slide open and one window in the kitchen, the tab is broken, I didn't know this because this was only the 3rd time in the unit and so I believe they got in that way, but I just don't understand the logic behind this, why would they break in just to break some bulbs and probably saw my screwdriver and figured why not take it. I can't prove it was them so I didn't even bother calling the cops. I took some long screws and screwed that window shut, replacing the windows is on the list of things to do anyway. Anyone ever have this happened right after a tenant moved out? Nobody else would have known it was vacant.