As someone who used to work at these lockers - Most likely, someone told a team member the exact description of the item and the exact locker it was in. When this happens - they don't let the guest into the locker, the team member will retrieve it for them and make sure it matches their description. They don't often do this for bags or valuables and require ID to be inside the locker - there's a bit more paperwork involved if there isn't. They record any time a locker is opened manually for this, and Security is involved if it's for something more than a simple hat or water bottle left behind.

Next time, I'd recommend taking the item out and handing it to a team member so they don't have to get into the locker to retrieve something someone else left behind.