Lambert does have witcher senses, so it's not impossible he wouldn't have picked Joker up if he was actively trying to sneak around ... but though he hears him approach, he doesn't acknowledge him until he actually speaks.

"The Carnival's fucking freezing," he says, bluntly. Of course, by now, Strange is mopping up that mess, so there's less snow on the ground, but yeah, you couldn't pay Lambert enough to go back there even now. Like with most of the Carnival's now-vampiric residents, he regards Joker with a touch of wariness, but otherwise not much else concern. So far, no one seems like they've snapped, but that can always change...

"This isn't really 'roughing it,' anyway." It's practically a paradise, honestly, compared to some of the places he's been.