Mainly because the kid ran around toward that other stairwell and is no longer near you with several walls between you. Something always blocks Detect and they are 'sort of' based off of your senses (in this case, the mental sense group), mainly things like range... I don't believe we've defined what 'blocks' John's detect. And technically, you have to use your Mind Scanning or Mental Awareness (which I don't think you have) in order to use Detect since you need to use the appropriate sense to detect. You mind scanned the building so I am fine with the detect if the kid were still in range. There are not automatic Mental senses that a character gets (like sight, sound, touch, etc)
As for the END... this isn't combat, you get a recovery every 12 seconds, 5 times a minute... so any END you've used will recover quickly unless you need to recover stun (and of course, you can choose which you recover)
I will say this, the description of Detect and how its used is pretty minimal in Champions Complete. It is fleshed out much better in the 6e books (well, almost everything is)