Hello and welcome to JustAnswer!
An intermittent battery draw can be a very tough thing to diagnose. What they've surely done is set up an ammeter to measure the amount of draw after the car goes to sleep, and it sounds like they've found the problem is no longer there. It may not return again at all, or it may be an intermittent problem that could act up every couple days or even weeks.
It's not unheard of for a control module to stay awake when the car should go to sleep. This will not only draw power because it is staying awake, but it can also cause other modules to stay awake because there is still communication bus activity. A power cycle will often bring a locked up module back to life, so when the battery dies the problem can go away.
Installing a data recorder on the car will allow them to record data from modules on the car and then play that back at the shop, basically it allows them to see much of the data that they could on a scan tool but it allows you to keep driving the car and hopefully get it to act up. There is a better chance of it acting up while you are using the car as you normally do than with it sitting in their parking lot.