Is it possible the sentry boy survived a huge assault with super mutant but had both of its main weapons blown off and were found to expensive to repair along with damage to a personality matrix to make it a bit off the wall could be a good reason to give it to delta.
Raises the question why not just scrap it and use if it has anything useful for other projects? Some Eyebots were scrapped in favour of resources for the Hellfire Power Armour. Still I'm sure that with the ongoing commotion the idea of scrapping a still functional Sentry Bot isn't to be done just in case weaponry for it can be acquired or vise versa for another Sentry Bot in need of something the one from the Sentry Bot Delta Squad was given.
It's finally here! I hope you enjoy! http://forum.eternalcrusade.com/threads/illinois-wateland-ic-thread.53201/
A quick warning, the IC thread may be taken down as I screwed up the title. No mods on right now, so it will have to be later... unless you're not bothered by it and want to explore the Watelands?
I completely forgot that when the Brotherhood of Steel in the Mojave went into hiding and lock down they sent up people from time to time to collect supplies. That could easily work for Maggie's character and while out there acted independently. If Maggie wanted it of course. The Enclave would have sent up scouts from time to time to see what is happening, gather some supplies they needed such as scrap that could be salvaged for repairs and create new equipment.
That guy in the robe at the gas station in Navarro wasn't the only one? We're lore compliant? In truth, running into other Enclave members will be fairly common for plot reasons. How would you like to join The Enclave?