Any ideas what these signs on this building mean?

Those are hazardous chemical markers. You commonly see them on tanker trucks as well.

The numbers range from 0 to 4, with higher numbers indicating more risk. The red top corner is flammability. The right yellow corner is instability; How likely it is to react with other things around it. The left blue corner is risk to health; Even if a chemical isn’t unstable or flammable, it can still be hazardous. The bottom white is for special markings. In this case, one of those chemicals is marked with a W, meaning it reacts to water.

So if there’s a fire at the warehouse, this tells the responding crew “hey just so you know, there’s some nasty shit in here. One presents a severe health hazard, becomes potentially explosive when heated, and reacts with water… But at least it isn’t flammable. The other is flammable and can present a moderate health risk. Because of the one on the left, it would be a bad idea to use water to fight this fire.”