Clues:
-There's no way to know how long it took them to get into the shed. You were asleep. You only woke up when they slammed the door shut and drove off. They could have been there for an hour, working on the lock.
-They cold have done it quietly just by taking their time.
-A hairpin is not a very effective tool for opening a padlock, despite what TV and movies show. However, with a proper set of picks, they could have opened a padlock with just a little work. They could have tried it with a pin, then given up and used a lock pick set, which they took with them when they left.
The big question is, how did they get all of that stuff into their vehicle? They must have been driving something big to hold a lawn mower, deck chairs and whatever else would be in a shed (chainsaws, weed whacked, cans of paint, a ladder, etc).
Here's my guess: The thieves had a box truck. They worked for who knows how long to quietly open the lock, then lift and carry everything to the truck. The "SLAM" was the sound of the rolling door closing, instead of the shed door opening.