Have you checked to see if it's adhered by "static cling" (tried peeling it away)? Ditto that an actual & better image along with the one you posted that gives perspective of its location would be beneficial.
Consider that it might just be a simple parking decal that specifies a special lot for a local college/university, sports-stadium, club, or parking garage floor for a specific business etc.. See images below of parking decal designs companies/universities use to facilitate their large parking needs:
Examples of Parking Decal Requirements
At the entrance to each parking lot there are signs indicating which type of permit the holder must possess in order to park in that lot. The parking permits and lots are color-coded as follows:
Parking decals are to be placed on the outside of the driver's side rear window
***************************************
Q: How should I display a resident parking decal parking permit?
A: Resident parking decal must be permanently
affixed to the inside top left corner of the front windshield...see example above. Every effort should be made to ensure that the decal or permit is visible from the outside of the vehicle and it does not block your view.
***************************************
Decal Placement
Vehicles:
Parking decals must be displayed on the left rear bumper (driver's side) or
bottom left corner of the rear window of the vehicle where the decal is clearly visible. Parking decals must be displayed on a rearward facing portion of a motorcycle so the decal is clearly visible.
**************************************
Cars parked in lots around Eagle Landing that aren’t designated for student parking will be towed beginning Oct. 1.
Cars with green decals will be confined to the third, fourth and fifth levels of the Eagle Village parking garage, according to an e-mail sent out to students from the Offices of Public Safety, Parking Services and Residence Life.
All student cars on the first and second levels, in the surface lot around the garage, and those parked overnight in the Eagle Village development will also be towed.
************************************************** *
Decal Display and Placement
Decals, with the exception of Temporary Permits, are to be
affixed to the outside of the driver side rear window/bumper.
*********************************************