You are correct that the best practice in any mapping exercise is to capture the entire storage area by placing your sensors at the limit of the usable space. This is an easy concept to apply in small units such as refrigerators and incubators. But even then, a similar discussion comes up: If I map a 1 cubic meter refrigerator, with my corner sensors 10 cm from the adjacent sides, am I obligated to never place my sensors out of these bounds? In practice, it’s hard to prevent the users from placing items outside the mapped space of a refrigerator. And we can all probably agree that the 10 cm doesn’t make a difference.
In a large space, such as a warehouse, the challenge is the opposite. We will usually need to store things on top of the top racks, but there is no easy way to get our sensors above the top rack safely, especially in a warehouse that's in use.
You are again correct that the only way to confirm 100% that the product storage locations on the top racks are within the temperature specifications is to get sensors up there.
I will offer these three ideas on how to resolve the issues posed here:
1) Anytime I have ever mapped a warehouse, we have put the sensors up as high as we could on the racks. And in most instances, this meant there was product storage above the sensors. This is one of the real-world facts that don’t always fit with the perfect world we’d like to see in our studies. So, your contractor is more or less correct.
a. (There have been a few instances where we did place sensors above the actual storage area by placing the sensors on the ends of poles. But the data was not convincingly different, except a few cases where the sensors were placed near HVAC vents.)
2) You have data from the top plane. If it is well within temperature limits, then you have nothing to worry about. If it is very near the temperature limits, then perhaps you have something worth investigating. But then the issue is not with the sensor placement, it is with the warehouse controls.
3) If you were to be audited and there was a problem found with the sensor placement, it wouldn’t be a big issue. You are ahead of the game by even mapping your warehouse. Despite it being a GMP requirement, many companies fail to comply with this. At worst, I expect the auditor would ask you to merely place the sensors higher on your next scheduled revalidation. It would be extremely surprising if an immediate revalidation were demanded.
I hope this information is useful in your decision-making process.