For all I know, they just did it for a lark.She came in late because she had an accident, for all I know.
for all (one) knows
Said when one is uncertain or has limited details about something. I haven't talked to Marie in weeks, so, for all I know, she's not even coming to the dinner party tonight.
for all I know
according to the information I have; I think; probably. (Usually implies uncertainty.) For all I know, the mayor has resigned already.She may have gone to town for all I know.
for all I know
phr. as far as I know; I really don’t know. She came in late because she had an accident, for all I know.