I was casually glancing over the Group of Soviet Forces in Germany wiki and read that some Soviet/Russian forces remained until 1994. This is obviously years after both German unification and the collapse of the USSR.
In what capacity did Soviet/Russian forces remain after German unification and the collapse of the USSR?
The answer is actually very mundane. From the Wikipedia you showed there were about 600,000 Russians in Germany in 1991. All of those people have personal belongings and need a home to live in. It's not easy finding a place to live for 600,000 people. In fact it takes about 3 years to find a place to live for 600,000 if you aren't going to just throw them in refugee camps. On top of that 600,000 people have records that need to be transferred. And on top of that 340,000 Russian soldiers have a lot of tanks and radios and guns and rations and classified documents and trucks and filing cabinets millions of friggin filing cabinets filled with paper. And 600,000 people have leases and speeding tickets and other matters to settle. And 600,000 people have secrets to cover up. Uh oh Russia have you been dumping waste fuel and oil into German lakes and Rivers? The only correct answer is "you can't prove we did."
The answer to your question is that all those Russian troops spent most of their time packing up and moving. Because it takes a long time to move 600,000 people.
Did I mention millions of filing cabinets. I visited a Russian Barracks after they left. Empty spaces for filing cabinets everywhere and right next to those empty spaces for filing cabinets were filing cabinets that had been left behind. I've never seen anything official about it but I suspect a large part of Russia's defensive strategy against nuclear weapons in East Germany had something to do with filing cabinets.