unbeknownst
adverb formal uk/ˌʌn.bɪˈnəʊnst/ us/ˌʌn.bɪˈnoʊnst/ (also unbeknown, uk/ˌʌn.bɪˈnəʊn/ us/-ˈnoʊn/)
unbeknownst/unbeknown to someone 在（某人）不知道的情况下
without a particular person knowing
Unbeknownst to me, he had rented out the apartment while I was away. 他趁我不在的时候把公寓租了出去，我根本不知情。