Bees will definitely find them - so will ants, wasps & bees from other apiaries. You can either prop the Flow super with frames in it against the front of your hive, or put it above the inner cover/under the lid so only your colony has access. Either way, be sure to take it away in a day or two - otherwise if outside it’ll increase chances of attracting the wrong kind of ‘cleaners’ 
and if inside your bees will start propolizing it.