From Longman Dictionary of Contemporary Englishunpromptedun‧prompt‧ed /ʌnˈprɒmptɪd $ ʌnˈprɑːmp-/ adjective formal WILLINGsaid or done without anyone asking you to Quite unprompted, he offered to help.
Examples from the Corpus
unprompted• All four heads took the unprompted view that there was a desperate need for professional library staff in secondary schools. 