I was also hoping in a patch we could get an icon to show workbenches that we have stuff stored in so that if this happens we can easily find them and especially for the ones that are not a part of settlements, I was close to starting a new game when I realised how much I have lost due to stupidly storing junk and weapons in random workbenches. I was lucky to find one of them again at fort Hagen and retrieved my items in the bench there.
I understand that was my own wrong doing to leave them there but there is obviously an issue with the settlement benches not showing us things stored between each settlement and yes the beach is mine as I done the quest and can build there so if it was not a settlement that I owned I would not be able to create supply lines or build anything for that matter.