I started collecting hidden mickey pins in 2008/2009 at
Disneyland.
I believe for the ones at DLR, they started with the 2010 series. Surprisingly, the completers came out in the latter part of the year, but that was also back when DLR released a new hidden mickey set each month, so each month one would go running around the park looking for that month's new hidden mickeys.
One good resource for pins is Pin Trading Database.
www.pintradingdb.com
What was interesting was that some of the series of pins had two completers, like these:
https://www.pintradingdb.com/pin/14302
https://www.pintradingdb.com/pin/14303
https://www.pintradingdb.com/pin/8371
https://www.pintradingdb.com/pin/8372
The train is obvious, since with the other 5 pins, it completes the Casey Jr. Train.
I stopped collecting hidden mickey pins a few years later, except for the series that I like, but I believe they have done completer pins every year thereafter.