That would be problematic to name in pecatore cardinals that were behind the iron curtain because as soon as they were named, the the soviets could get and execute them. There has to be some Church law governing a situation like the Soviet Union where it would be detrimental for the Church to name or "out" the in pecatore cardinals in the areas.
The Pope is the only person who knows the name of secret cardinals. If he dies - having left it written down for his successor - the successor can simply ignore it. Therefore, it is most just that the cardinals not named not be cardinals after the death of the Pope. Anything else can cause controversy (as in the case posited by the subject of this thread).
If the Pope wanted an iron curtain cardinal to remain so and he knows he is dying, he would need to bring him to a safe place and announce it.
Also, note, when the Pope does make cardinals in pectore, he states officially the number of them. Pius XII made no cardinals in pectore.
Of previous Popes who did, here are some examples:
Pius X made one (secret due to the revolution in portugal) - he was named shortly before Pius X's death.
Benedict XV and Pius XI each made one and both were revealed.
Pius VI declared 3 - none were ever named.
Pius VII named 11 - all were revealed DESPITE the revolution in France (the reason he created them).
Gregory XVI made 29 but six remained unpublished.
Again note - if the Pope does not announce that he has created them - he hasn't. So even if Siri were Pope - he never announced the creation of in pectore cardinals.