Although in principle all search work (other than for Art. 54(3) material) should be done at the search stage, in exceptional circumstances examiners are not barred from looking for a relevant document whose existence they know of or have reason to suspect, if they can retrieve that document in a short time.
Source: http://www.epo.org/law-practice/legal-texts/html/guidelines/e/c_iv_7_3.htm
Date retrieved: 17 May 2021