I think it's been raised in the context of a fair few sightings where there's supposedly been a very large craft moving silently and relatively slowly, e.g. the

I think it's been raised in the context of a fair few sightings where there's supposedly been a very large craft moving silently and relatively slowly, e.g. the 2000 Illinois sightings (referenced in a great song by Sufjan Stevens). I was looking into this one a few years ago and found references to a private company testing blimp platforms for military purposes around that period, although I can't find it off the top of my head. More recently, the object spotted hovering off Hawaii last year that resulted in fighters scrambling was also proposed to be a modern balloon-based drone, of which there are a few currently being developed. Edit: here's a source arguing that the Illinois sighting was a regular advertising blimp https://skeptoid.com/episodes/4435 https://skeptoid.com/episodes/4435