On the topic of garbage
In the 1998 Supreme Court decision of California v. Greenwood, it was found (among other things):

It is common knowledge that plastic garbage bags left on or at the side of a public street are readily accessible to animals, children, scavengers, snoops, and other members of the public.... Moreover, respondents placed their refuse at the curb for the express purpose of conveying it to a third party ... Accordingly, having deposited their garbage ... in an area particularly suited for public inspection and, in a manner of speaking, public consumption, for the express purpose of having strangers take it ... respondents could have had no reasonable expectation of privacy in the [items that were] discarded.^{[findlaw.com]}

Furthermore:

The question, then, becomes whether placing garbage for collection constitutes abandonment of property ... [T]he placing of trash in garbage cans at a time and place for anticipated collection by public employees for hauling to a public dump signifies abandonment ... [T]he act of placing garbage for collection is an act of abandonment which terminates [any protection]... The act of placing [garbage] for collection is an act of abandonment and what happens to it thereafter is not [protected].^{[findlaw.com]}

In the 1995 case of United States v. Hall, United States Court of Appeals for the 11th Circuit, it was found that:

It is common knowledge that commercial dumpsters have long been a source of fruitful exploration for scavengers. ... [The defendant] did not take sufficient steps to restrict the public's access to its discarded garbage; therefore, its subjective expectation of privacy is not one that society is prepared to accept as objectively reasonable..^[emory.edu]

Additionally, summarized elsewhere:

Stealing trash is not illegal. The Supreme Court ruled in 1988 that once an item is left for trash pickup, there is no expectation of privacy or continued ownership.

By definition anything in a dumpster is waste and therefore not needed and abandoned by the owner. So you can recover anything from the trash that you want.

The problem is the location of the dumpster. If the dumpster is on private property then you can't access it without trespassing.^{[answers.com]}

It would of course be perfectly reasonable for an occupant of the building to access a dumpster that is provided by a building for the express use by its business tenants.