I reckon it is somewhere within the mysterious mechanics of the sofa bed.
When we last moved from one house to another the removal men, for ease of getting it out through the doors, took our sofa bed apart ..... we found all sorts of stuff that had been missing for years!
Kids toys
A lot of bread/biscuit crumbs
Several pounds in loose change
A pair of knickers (that no-one would admit to having ever seen before)
A single stocking (ditto)
Other items too delicate to mention here but "Dongle" would be an apt description.