Fact: Special obstacles
do and
have spawned on "dead and abandoned bases".
Fact: Special obstacles are
not limited by the total obstacle count limit.
Fact: Normal obstacles
are limited. Hence, that ensures that your base will
not become so crowded with normal obstacles that there is no room available for special obstacle spawns. Include in that consideration that if you walked away for 6 years, you have a lower TH level with lots and lots of room.
Conclusion: The obstacles
did spawn on your base over the years, and you
freely elected to harvest them for resources over those years.