I was traveling with Piper, I was switching settlements so I gave her all the equipment I was carrying, I fast traveled back and forth transferring loot until I figured I'd tell Piper to stay at the new settlement (Dismiss her and send her to Drive-In). So after some adventuring I realized I'd forgotten to take back the items I'd given (my best weapons and armour), so I went back to the Drive-In and she isn't there. To try to get her to spawn again I have: waited more than 3 days, installed a bell to attract all settlers to one spot (she doesn't show up), checked every other settlement I've discovered (I know she is at the Drive-In because the settler count is at 6 and I ring the bell and only 5 show up so I know someone is missing), checked every square inch of the settlement at least three times, fast traveled to and from the settlement 15+ times and still I cannot find Piper. If anyone can give me some kind of method to locate her immediately, some way to mark her position on a map, force her spawn or anything that would be greatly appreciated, this is really bothersome seeing as some of the items she was carrying were Legendary making the likelihood of coming across them again virtually impossible.

This is just a guess (and I haven't met Piper yet myself), but try going back to where you first met her (not the settlement - I believe maybe Diamond City but I'm not sure since I haven't been there). I've heard sometimes companions return to where they start from.