I'd be surprised if they could detect battery swelling on a scanner. The battery's horizontal area doesn't really change, it just kind of bulges. The scanners are two dimensional and they make you lay your computer flat in a separate bin so it's not like they could detect vertical swelling.
I also had a similar reaction to my 2014 rMBP once (and only once, over a few dozen flights). I figured MBPs just look scary from the wrong angle, or maybe new scanner employees who haven't figured out what all the common laptops look like yet.
Maybe it's all the little "terraced" sections the battery cells are cut into. If it's your first time seeing that as a scanner employee, it doesn't really look much like any other "battery."
One time at LGA, there were 4, 15" MBPs going through the X-ray in a row. Mine was #3, and it got pulled for additional screening. When I asked why mine, they're all the same laptop and the guy just shrugged.
Leaving KEF a few weeks ago, my bags were pulled for a random check. But, at least the Icelandic security guy was nice enough to tell me as much...
"Your bag was selected at random for an inspection."
"Now, I'm going to carry it over to that table. Please follow me.
"Now, I'm going to unzip it."
"Now, I'm swabbing for residue."
Quick, professional, and not at all stressful. Unlike similar experiences at IAD and FCO.
I also had a similar reaction to my 2014 rMBP once (and only once, over a few dozen flights). I figured MBPs just look scary from the wrong angle, or maybe new scanner employees who haven't figured out what all the common laptops look like yet.