Unnerving morning

~~on my lunch break~~
I arrived at work this morning to find my inner office door ajar. I was quite sure that I had locked it last night when I left at 4pm. A quick glance around and everything seemed O.K.
One of the techs who is at work early commented that my door was open when he arrived and there was a terrible stench "like a dead squirrel in the wall" in my office. Thankfully I was spared that.
Then...the chap next door said when he left at 5:30 pm last night, our outer door was open and when he came in, calling out to see if anyone was here, he discovered my office door open and the warehouse door unlocked and unbolted and the lights on. Curiouser and curiouser as the owner's son assured me that, "Everything is locked," when he left around 3pm.
While we were hearing this I opened my little fridge to put my lunch away and discovered my left over salad from yesterday and my apple juice was missing!! What is going on? Then "the boss" discovered the bottom of the salad container on the floor in the men's washroom. yuk (no I do not want it back)
Yesterday was a mild day and our office and the business next door had our doors propped open to the foyer and the outer door open as well. What the men think is that sometime after 3pm someone came into the outer office snuck past my door while I was on the computer ( I face the window) and hid in the warehouse while I was at my desk. Are you freaking yet because I am? Normally I check the warehouse before I leave but as I had been assured everything was locked, and had no reason to doubt it, I skipped that part of my routine. I locked my office door, shut off the lights, locked the outer door and went home. I locked the intruder in. Oh crap. It is assumed that the "person" then let himself out and was probably planning on coming back with a truck sometime in the night and cleaning out the warehouse. Our neighbour who has a set of emergency keys locked up thinking that I had lost my mind and gone home leaving the whole damn place open for the asking! Jesus, Mary & Joseph!!!!!
I am alone in this office 90% of the day. To say I have been jumpy today is putting it mildly. I am getting a motion sensor for the front door.

Now here is the mystery. How did he get in my office? Today I am not happy.

edit: A new motion detector has been installed at the front door. It chimes three times when ever someone comes in or out. I do not mind it one bit!