Hi, how can I make this function return her default behavior?

private object CanUseLockedEntity(BasePlayer player, CodeLock codeLock)
{
    if (!codeLock.whitelistPlayers.Contains(player.userID))
        return false;

    return null;
}

The above allows you to avoid unnecessary hook calls, casting, and returning.

here is what I get if I try to return null:

Error while compiling: Plugin.cs(70,20): error CS0037: Cannot convert null to `bool' because it is a value type

Oh my fault, I didn't know I could "edit" the hook:

from: private bool CanUseLockedEntity, to: private object CanUseLockedEntity

You could either follow Wulf's example or use the nullable bool type "bool?"