be anybody's/anyone's guess
To be uncertain or unknown. Why dogs and cats hate each other is anybody's guess. No one has been able to determine why my daughter has been having nightmares—apparently, it's anybody's guess.
be anybody's guess
or be anyone's guess
If you say that something is anybody's guess or is anyone's guess, you mean that it is impossible to know what is true or what will happen. How she managed to stay there for an entire month was anybody's guess. How long the fuel would last was anyone's guess.