Essentially, The Word "Nioi (匂い)" In Japanese Literally Translates To 'smell,' But It Is Also Used

Essentially, the word "Nioi (匂い)" in Japanese literally translates to 'smell,' but it is also used subjectively to express 'guessing' and 'certainty' when sensing something. In other words, the term "detective's smell" is used to emphasize the strong intuition of sensing something detective-like. Considering the original Japanese text, Grimmer's remark to Runge that he smells like a detective can be most appropriately interpreted as "he gives off a detective vibe, almost certainly." However, I still believe that there can be 'ambiguous' interpretations of this statement. In other words, Grimmer deduced that Runge was a detective based on his words and actions, but it's also possible that Grimmer was literally close enough to smell him. After all, "Nioi" is still a word that refers to smell. In that sense, I wholeheartedly agree with your opinion!!

Grunge

I hate saying stuff, because I always regret it in the end. But once in a blue moon my desire to say something grows so strong that I crawl from under my rock and make some sort of a statement somewhere. It's that time now.

I've already talked my friend's ear off with this crap, but it's still not enough. It also drives me up the wall that I don't see this stuff being said all the time, or at least some of the time. I guess I don't see it being said at all. So I guess I have to say it myself. 

I'm talking about this huge thing Runge canonically has for Grimmer. Not even in the romantic sense... Let me explain. In the crudest of terms, too.

By the time Runge makes it to Ruhenheim, he is already not quite the same as he was at the start. His path to Ruhenheim was the most tedious, and yet he still got there before everyone else. And Grimmer definitely catches his eye when they intersect. Again, not in the romantic sense. They just go around the town, exploring the surroundings and kinda watching each other. And they both know. They know what's going on, they know who the other person is, but they just waltz around it for quite some time. Until they finally have a proper conversation in that desolate place. And that conversation, man...

They understood each other with half-word, literally skipping a good portion of talking, because that would just be redundant. Runge is absolutely chill with the idea that Grimmer might be a murderer. The guy has a nice smile, so who cares, right? Grimmer states that Runge "smells like a detective". Is it just me, or does that sound, uhhh... extremely weird? Almost in a flirty sort of way. Because you gotta get pretty close to someone to take a good whiff of them. And hearing something like that being said about yourself, it must be pretty embarrassing, or confusing, or thrilling. What does that even mean, by the way? I thought so hard about it, and I still can't tell for sure. What do detectives smell like? Gunpowder, paperwork and cold, hard determination? Do they reek of sweat, or some special kind of cologne? I have no idea. But I guess Grimmer has, and Runge, he just stands there, looking at him like it's a given.

And after that, they just click. "Oh, hey, we're on the same page, we're after the same thing". And without any negotiation, they simply decide that they work together now. I am fully convinced that at this very moment Runge went, "This one". No context, just – "This one", about Grimmer. 

He then proceeded to entrust Grimmer with his most important evidence, the very one that led him to Ruhenheim. And then, at Versteck... Man. His "I'll buy you a beer" says so many things, just because it came from Runge. He doesn't have any friends. He doesn't even communicate with his colleagues properly. But, somehow, he is still willing to bond with Grimmer. I guess it's a "we're the same, so let's be alone together" thing. At the same time, he's just heard Grimmer stating that he cannot enjoy simple pleasures in life. That means, he wouldn't be able to enjoy this suggested after-work-beer either, right? So Runge's invitation serves as a double promise: that they'll get the job done and make it out of Ruhenheim alive, and that Grimmer will be fixed enough to finally enjoy that beer with him. 

Needless to say, Runge really, really wanted to keep Grimmer alive. And he doesn't cope with Grimmer's death well. Say what you will, but visiting someone's grave with a beer after A YEAR that they've passed, lamenting how you wish you would have known them, is pretty damn strange. And I suppose Runge might've visited Grimmer's grave prior, too, with some periodicity. The card, the plans, the beer, the hinted promise – he really wanted Grimmer to become his friend. And just because it's Runge, who refused to even engage in a casual talk with his peers and barely cared that his wife and daughter abandoned him, this thing he has for Grimmer seems to be especially large, unalloyed and special.