Change the method signature
Probably that plugin was created for an earlier version of CakePHP. The error reads:
Strict (2048): Declaration of SearchableBehavior::beforeSave() should be compatible with ModelBehavior::beforeSave(Model $model, $options = Array) [APP/Plugin/Searchable/Model/Behavior/SearchableBehavior.php, line 5]
I.e. in the searchable behavior there is this:
public function beforeSave(Model $Model) {
And instead it should be changed to
public function beforeSave(Model $model, $options = Array) {
That kind of warning will always be shown whenever a child class redefines a method and give s it a different signature.