Anyone who actually believes united come out of this with any credit is irredeemably thick.
Whenever a commercial pilot sits in his cockpit prior to taking off he should never for a moment forget his responsibility to the people who are in his charge for the next few hours. That means adhering to procedures, double checking everything and fully concentrating at all times. It's what he gets paid 80 grand a year for.
Football clubs have a similar responsibility when tens of thousands of supporters converge on their ground. They must take every available, reasonable step to ensure those supporters' safety. This has to include a thorough search of all public areas prior to allowing entry to the ground, as this step is both entirely reasonable and readily available to them. How could anyone suggest otherwise? The fact that a terrorist threat is reasonably foreseeable further accentuates their culpability.
united have miserably failed in their duty of care to those that trust it with their safety. The fact they have sub-contracted part of their security regime is a complete ignoratio elenchi to that fact. As owners and operators of that ground it is their responsibility to take all necessary steps to ensure it is sufficiently safe, which they have conspicuously failed to do by failing to spot this device, irrespective of its purpose. I fail to see how anyone could argue otherwise with a straight face.