Researchers browsing open scholarly resources on Gallica, the digital platform of the French national library, need to tinker with its dysfunctional search engine in order to make discoveries. While discovering unknown, yet relevant scientific resources is of paramount importance for knowledge making, little is known about how researchers deploy specific informational practices on, usually ill-equipped, digital scholarly platforms to turn search engines, designed for findability, into discovery engines. By crossing semi-directed interviews with log analysis, multiple "regimes of engagement" with Gallica were identified. Building on pragmatic sociology, the regime of unstructured exploration as a strategy for users to venture into the "noise" and make discoveries was studied and showed the inherent tension between findability and discoverability, heightened by the hegemony of general search engine. This mixed-methods ethnography of discovery practices on a digital library can help design specific tools and scientific policies.