This sign was posted near an area where someone was doing rat extermination. This was done through rat poison boxes, and I would guess that laws require the poisoners to inform the local public of such an event. But I think that this warning was much more than just a generic warning! Especially as these warning signs were not posted not only near the particular target area, but next to any heavy traffic areas close by like next to a bus stop.
We think the meaning of the sign, ostensibly aimed to be a "legally mandated warning" was serving the secondary, or primary, purpose of communicating that "There is a need for rat extermination in this area. (Perhaps you need it too?)". This seems plausible, especially when looking at the sign design; the rat has red eyes, sharp teeth & claws and halfway out the bounding red triangle it is maybe gnawing at. This is clearly an evil out-of-control rat that is a threat to the viewer in particular.