His first clue had gone something like this:
Sam, Dean and Cas were sitting around the large table in the library at the bunker. No one had said a word in hours and by now the silence had strongly rooted itself between them and none of them were willing to disrupt it. So for the last few hours the only sound they'd heard was the occasional scrape of paper on paper when someone turned a page in whatever ancient text they were reading.
That is until an ear splitting smash caused the table to rattle violently and both brothers to jump a foot in their air, hearts pounding and they looked around wildly for the source of the noise.
Turns out the source was Cas. The angel had closed the six inch thick tomb he'd been pouring over and smashed it down, with what had to be all of his angel strength, onto the table.
Or at least what was left of it. The wood was cracked and splintered under and around the area of impact and the book itself had come down with such force that it was now a part of the table.
Once he felt he could breath again and was feeling a little light headed as the adrenaline wore off, Dean moved his hands out to their side as he stared, wide eyed, at the psychotic angel.
"Dude. What the fuck?!"
The angel looked up at them both as if just remembering they were there and had the decency to look slightly embarrassed.
"Spider." Was all he said.
Dean looked over to his brother, wondering if he was missing something. If he was, Sam would be the one who put the pieces together. But no. His brother looked just as alarmed and confused as he likely did so he straightened up and cleared his throat, arranging his face into something he hopped would pass as calm and collected.
"You know what, we've just been reading too many dusty old books for way too many hours and we just need a break."
"Uh, excuse me, I did not just obliterate half a table because of a bug." Sam said.
He cuffed his brother over the head with a ground, "Not helping."
When he looked back up, Cas was tugging at the edges of the book, trying to pry it from where he'd embedded it into the table top but the tomb did not want to budge. He almost laughed when the angel paused to scratch the back of his head as he started down at it helplessly.
The second clue came a few months later when Cas was tagging along with them on a hunt. They'd just stopped at a not-as-gross-as-normal motel and Dean was checking them in. The guy behind the counter was greasy and creepy as fuck, with a stringy black pony tail and a seventies porn star moustache that looked like it hadn't seen a comb in about three years.
And right up on the counter, where paying customers stood, was a small glass aquarium with the biggest, hairiest spider he'd ever seen sitting happily on a rock under his heat lamp. He wrinkled his nose at the creature but couldn't help tapping the glass curiously while the guy had his back turned to make change.
He heard Cas' steady footsteps come up behind him and turned, "Almost done."
"Sam said to tell you-" The angel cut off abruptly and Dean looked away from the greasy receptionist's back to see that Cas' blue eyes were wide and he was staring intensely at the tarantula in it's glass enclosure.
He looked between the spider and the angel a few times and when the clerk handed him his change he thought he saw Cas' right eye twitch ever so slightly as he continued to stare at the spider cage.
"Dude." Dean said, snapping his fingers in front of Cas' face to get his attention. "Let's go."
No sooner had they gotten three steps out the door than Dean heard a shout of surprise followed by a distressed, "HOLY FUCK!" that had him whirling to see the greasy clerk frantically but uselessly blowing on his tarantula tank which had just spontaneously burst into flames, shards of glass littering the floor around the counter as if a tiny explosion had gone off inside the tank.
"Shit." He muttered. Then frowned. "How the hell did that happen?"
"Probably the heat lamp." Cas said disinterestedly. "Come on, Sam want's to get started."
He watched Cas walk away from him and back over to where Sam was leaning against the impala, waiting to see what room they'd be in, and felt that niggling sensation in the pit of his stomach that let him know he was missing something obvious.
The final piece of the puzzle went something like this:
Dean pulled a few cans of soup with the word 'vegetable' in the title just so Sam wouldn't bitch and moved on to the bakery section where he found his brother looking at a wall of about a gazillion different kinds of bread.
"Where's Cas?" He asked, noticing that the angel was nowhere to be seen.
"I left him at the sea food counter 'cause he was arguing with the guy about whether or not crabs have souls."
Dean sighed. "Oh course he was." He watched for a moment while Sam continued to contemplate the bread choices before finally snapping, "Dude, pick one."
After a brief argument over the choice Sam made was resolved they decided to go find Cas and check out. They'd only glanced down one empty isle and were moving on to the next when a blinding flash of light had shoppers screaming and fleeing the store and the brothers staggering and trying to blink away the black spots in front of their eyes.
"Did he just-"
"Yeah."
"Did you see where it come from?"
"The cereal isle, I think."
They found the angel in the deserted cereal isle, staring down at a small, smoking, black speck on the floor with as much intensity and hatred as Dean had seen him smite demons.
"Dude, I think Cas has finally lost it."
He ignored his brother and approached Cas, smacking him on the shoulder to get his attention. "What are you doing, man?!" he cried.
Sam came to a stop beside them and stared down at the smouldering smudge, squinting as he tried to make out what it used to be.
"Cas. Did...did you just smite a spider?" He asked, looking down at the angel a litter nervously.
The angel merely stared at him, blue eyes impassive as ever.
"I don't like spiders." He rumbled. His eyes slid over to Dean. "They have a two for one sale on your favourite cereal."
And then he just walked away towards the front of the now deserted store, presumable to go wipe the memory of the manager that was cowering by the washrooms.
It was then that Dean realized – Cas was scared of spiders.