There are much more significant instances of Soviet retrenchment and retreat during this period. Probably the most notable is the withdrawal from Austria. The USSR held a significant amount of territory in Austria at the end of the war, with the US holding the rest, and quickly agreed splitting the country into 4 allied sectors of control, and doing the same in Vienna. Shortly afterwards, the USSR withdrew from Austria completely. Why? Well, Stalin was not one to share his strategic thinking with the public, so we can only guess. Probably he did not want to deal with a "West Vienna" along the lines of "West Berlin". But who knows.

As for Bornholm, I really don't see why people think it would have some great significance. The Soviets already had many bases in the Baltic which were contiguous with their territory. What would an island add? The Soviets did not have a good record of supplying their forces by sea. During WW2, i.e. immediately before the time in question, they suffered a number of disasters when their forces were cut off from supplies by land and had to be supplied, and then evacuated, by ships. There was the disastrous evacuation of Tallinn at the beginning of the war, the battle (and evacuation) of Odessa, the loss of Crimea, and the Leningrad front with immense civilian and military losses. Bornholm would have looked like a death trap to the Soviet commanders of the time, rather than some great asset. No wonder they agreed to return Porkkala to Finland 40 years early, and never placed naval assets of any significance there.