Given my experience with those who are not into "preparedness", my guess is the guy was embarassed to be caught "preparing". I rarely mention my preparedness supplies/stock to anyone NOT because I'm worried about losing it, but only because I'm tired of hearing comments like "that's weird", or "you're nuts", or "When's the comet coming", or "did you join a cult", or "paranoid", etc. etc. etc.
You get enough of that and you just want to avoid it. At least 98% of Americans have no clue where their water really comes from, how food gets into the store, how a gasoline supply chain works, where their poop goes when they flush, how few cops there really are, etc. etc. They are basically clueless, and rather than deal with it on a rationale basis they live in denial and mock those who happen to accidentally pierce their thin shell of ignorance. I really doubt he was hiding anything, just surprised and trying to come up with a plausible "cover story" to avoid telling you the real reason, which was "I'm worried about taking care of my family, and this gasoline could end up saving our lives."
Of course, this is all just speculation on my part - who knows, he might have been accumulating supplies for his local militia - maybe they were running low on Molotov Cocktails?