grammar - What is the noun in the second sentence?

This is a continuation of the question at A significant amount of zombies were detected in your city which raised further questions in my attempted answer.

There were purchases. A significant number of them was/were detected.

From [Oxford Dictionaries][1]:

Although the expression ‘a number’ is strictly singular, the phrase ‘a number of’' is used with plural nouns (as what grammarians call a determiner (or determiner)). The verb should therefore be plural:

A number of people are waiting for the bus.

This is not the case with ‘the number’, which is still singular:

The number of people here has increased since this morning.

It can therefore be argued that "them" refers to "purchases". Is "them", "purchases" or "a number" the noun in the second sentence, and why?

Answer

'A significant number' is a stand-in for what can only be imagined as a relatively large number. More importantly, that number of significant size would merit pluralization.

"1 was detected"

"2 were detected"

"A significant number were detected"

"A number [of something] were detected"

As the only number that leads to the singular case is 1, pluralization is the natural choice for unspecified quantities.

To answer your question, the noun is 'number.' The definition of number, per Google:

1.
an arithmetical value, expressed by a word, symbol, or figure, representing a particular quantity and used in counting and making calculations.

2.
a quantity or amount.

Considering the second meaning of 'number,' the pluralization is much more rational.

EDIT (after comment by Edwin below)

Plural noun usage is explained nicely here.

Relevant quotation:

Note that "the number" is a singular collective noun. "The number of applicants is steadily increasing." "A number," on the other hand, is a plural form: "There are several students in the lobby. A number are here to see the president."