Parking dilemma

My parking spot at work is inside the building, in an area reserved for only those who work in this building - may not work for my company, as there are other tenants. The parking area is tight - there are structural columns every twenty feet in all directions, and they have squeezed parking out of the smallest spaces in order to make more money. It’s difficult for me to get out of my parking spot and the rest of the garage for that matter if it’s not the end of the day, as many "longer" cars block my path.
Anyway, I came in on Monday and there was a car in my spot. And "my spot" means the one that I'm assigned, and not just the one that I like to take because I've used it for a long time. This really pissed me off as now I had to go find another spot without taking someone else's and causing another problem for myself. I could have left and parked in the parkway lot outside the building, but that's a big pain in the ass since there is only one door in or out of the garage, and there were a constant stream of cars coming in; I'd be waiting all day to leave. I called around to my coworkers and found a not quite so legal spot that someone who was on vacation usually uses.
I marched over to my spot and took a look at the car there It was a brand new grey elicipse with NJ license plates. I boldly opened the passenger door - unlocked - and went into the glove compartment to find out who it belong to. Chris or Lori Schwartz of Manasquan, NJ. Well it was no one that I worked with, it’s probably someone else in the building or some ass who just followed people into our garage thinking it was public parking.
Next stop building office. I complained about this strange car in my spot - but they were more concerned about where I was parking since I couldn't park there. I guess they didn't want a constant stream of people coming in as people parked "one over' from where they normally parked. They accepted where I temporarily left my car and said that they would leave a "note" on the car. What I about towing it away like they say on signs all over the place? Well, that's an empty threat, as they can't fit a tow truck into the garage when there are people there. So pretty much the car was there for the day.
My brain percolated all day along about this. Some guy was laughing at me cause he got my spot and made me run around. But what could I do? I thought about a brick / rock to a window, but since I had complained about it, I think they would know who to ask about it.. No, this needed a more subtle approach. After much deliberation, I printed out the following message
I DRINK URINE FOR MY HEALTH
and taped it to his bumper with packing tape - away from Mr. / Mrs. Schwartz's view as they would have gotten back into the car in the evening.
It brightened my day thinking of everyone looking at this car driving up the turnpike with that homemade bumper sticker.