。
She found her wallet under the desk.
Literal
She [topic-は] desk [possessive-の] underneath [at-で] wallet [object-を] found.
机の下 ('underneath the desk') uses 下 as the positional noun, paralleling 机の上 in structure but pointing the other direction. The で on 机の下 marks the location *of the action* (where the finding happened), distinct from the に that would mark the location *where the wallet sits* (e.g. 机の下にある 'is under the desk'). Japanese carefully distinguishes location-of-existence (に) from location-of-event (で). 見つける ('to find') is the transitive 'to discover, to spot' — its intransitive pair 見つかる means 'to be found, to turn up.'