I think many of us have been there, done that, and yes, it's a pain. If you kept your old orders around (or keep a plant purchasing log, which I've learned to do), that could be helpful.
Otherwise...
Make garden maps (not fool proof but so long as you don't plant similar irises together it can be somewhat helpful)
BURY a (waterproof label) marker with the plant
FYI, depending on exactly what kind of markers you had, the culprits could have been critters, not humans. I swear that the critters here keep going after my 4" plastic labels (which I use as temporary markers). Or it could be that the critters were digging around the plant (squirrels burying or searching for buried nuts), the labels got dislodged, and then got swept/blown away.
Metal labels... especially heavy ones, or ones that it takes some effort to get out of the ground.... I'd agree that those were probably ripped off by humans, for whatever reason.