Recently, this question asked whether you could Counterspell a spell without a material component that completely eliminates component requirement via Subtle Spell. The answer was no, with a Crawford Tweet as evidence. The following was asked in response to the Tweet but never answered and is a situation likely to occur in a campaign I'm running:
A Sorcerer uses Subtle Spell on a no-cost material component spell like Sleep or Fireball which they cast with a small crystal Arcane Focus kept tightly gripped and hidden in their fist.
In previous editions, spell descriptions indicated how materials were to be used: sprinkling sand or rose petals for Sleep, or throwing a ball of bat guano for Fireball. 5e doesn't provide these, nor does it seem likely such usages are possible with a Spellcasting Focus.
So, in this situation, is there enough information for another spellcaster to realise a spell is being cast for them to Counterspell?