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]:
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?