Oh my. i know EXACTLY what you speak of.
Once many many moons ago, my first ten gallon ever, i was like 15, running through pet co with what i had for money. my first 3 weeks into that tank, after having put in all my lovely gravels and decorations. even without a single live plant, i was finding on decorations this white eerie looking, sacks. full of nothing mind you, of the 3 i reached in and popped they never had anything in them and were extremely durable beneath water but more to the consistency of a sinus infection.
Ever since then though, through all my many tanks, (most of them planted some had only plants in the sump/refugium) i have never, ever, seen this. I thought it was just a fluke, some garbage i unknowingly picked up at my local deathco.
I wish to know, just like you wish to know.
What the hell that stuff was.
EDIT: mine weren't necessarily as shapely strict as yours. one (of the 3) was a perfect dome about the size of a dime, the other 2 had rounded bases no bigger then a pea but had a "sac" that extended approximately 1/2 to 1 inch in length.
Again using terms like "base" "sac" and even the word "thing" loosely.
