I noticed this happened in the past and when I checked my pets into the resort at the time I didn't notice the caption stating it would cost 0SP to check them in.

I checked in all my pets for 21 days, and charged me 0 SP for doing so, not sure if this is a Gold Account related bug or normal bug across the board.

On second glance, it didn't extend all the pets stays and their current days in remained the same. So possible GA bug with extending all the pets stays within the resort