One of the Doxies recognises him
As pointed out by Thunderforge, In the market square when K is having his noodles, one of the three Doxies recognises K and warns the others:
The THREE DOXIES cross the market to approach K. Flank him
at his table, studying the PHOTOS: The FARM. The BONES.
Dead SAPPER MORTON.
Hello hello A-Boy... You alone ?
DOXIE #3 recognizes him. TELLS THE OTHERS, DISGUSTED.
(in FINNISH, UNTRANSLATED)
Leave him, he is a fucking Blade
Runner... I’ve seen him. This guy is
dangerous. You coming?
It’s ok... I’m good.
-Final Shooting Script (undated), Blade Runner 2049 (2017), Fancher, H. and Green, M.
As for how he was recognised as a Replicant: they are now easier to tell apart
Luv tells K that in the past, Replicants were not so easy to spot as they are now. Speaking of Rachael and the Voight-Kampf test in the 'memory vault':
The IMAGE BREAKS OFF. REPEATS again in a LOOP. The eye
BLINKS, the voices come. A ghost fragment of an indelible
moment. Too brief. IT PLAYS OVER AND OVER AGAIN as --
It was unclear what she was, at least to
someone. This was a test. We were
difficult to spot then.
Now have a look at the way that Luv is described:
LUV. Polite, efficient. Perfect. Moral as a tornado and
about as safe. Beautiful, yes. The way a sword can be if
it’s safely behind glass.
Luv comes toward K. Hands tucked. Suit sensible. Every
aspect immaculate. The flawless representative.
Another prodigal serial number returns.
A 30 year old open case finally closed is
a curiosity and relief. Thank you,
Officer. I’m here for Mr. Wallace.
He named you. You must be special.
He steals a once over, understanding. She gives an
unembarrassed nod of confession.
Luv is 'perfect' in every sense of the word and K pegs her immediately.
Now note how a 'normal' human is described:
A pug COP, NANDEZ, enjoys the show. A bigot with a mouth
certain whatever comes out of it is funny. Recognizable as
human as all humans stuck on Earth seem to be.
[all emphases mine]