If i search in old local newspapers (soviet-time) about exploring or anything abandoned, the only thing i can find is some articles about collapsed bunkers and buildings after 2nd world / some old manors which are considered cultural monuments / abandoned houses which should be included in kolchoz areas. That's it. There weren't that much things to explore then.
After Soviet Union collapsed, thousands of buildings in Latvia became abandoned. All the army bases, civil defense shelters, some factories, a lot of kolchozs, cinemas, culture-houses, KGB facilites, sanatoriums and so on. There was so much left and so little that actually somebody needed.
The first purposes of exploring here were scrapping and...seeking for drugs. A lot of junkies and youngsters wandered in factories and old bunkers to find tablets which were originally produced for anti-radation sickness. These tablets can cause strong hallucinations and addiction. These tablets were freely accessible in abandoned places but some people tried to sell the tablets wherever they could.
People were curious about abandoned army bases because it was impossible to even get close there back in the soviet times. A lot of people explored because they were curious about how the places looked, what was left after...
In 90s newspapers wrote that it's too expensive to guard public buildings if the building has no owner or whatsoever.
I'm sure that there were people who explored for fun in the 90s but that was super-underground, there are no newspaper articles about those people.
In 2003 one movie about a roofer was made here and it's protagonist said: "I explore rooftops in my free time and i have like-minded friends."
First articles in newspapers about people who called themselves as an "explorers" here(Latvia) appeared only around 2006(!) and only in 2007/2008 such local activities became really "visible" on the internet,newspapers and other media.
It's just very interesting to compare how differently something started in post-soviet countries. But i totally agree - exploring is as old as mankind. Children especially would explore any nearby surroundings because it's just natural.