Then I'd get to checking said boiler and local attached bits for a leak, pronto.

Puddle of colored fluid gathering underneath something as dangerous and important as a powerplant boiler would/should be cause for huge alarm, not "huh, neat. Wonder what that is" :P

Though the more I look at it the more it still looks lime a vehicle was parked there and leaked. Can see what looks like a tire track leading up to the smushed bag (less tread mark, more even width path lacking dirt amd sand)

Looks like it was from something dripping down from above that made a puddle, then froze and started making the pole in the center.

I'd assume coolant from a vehicle that was parked there, assuming there isn't something like a pipe or machinery out of frame over the site.

Also checks out with that insulation bad in the top right that looks like it has a dent the size of a normal tire, like someone pulled up to far when parking and ran into it slightly. Would also then put the puddle around the side of a radiator or coolant tank.