"Oh, sorry. I'm Yffub, the Ranger Who Has Chosen Undead to Be Her Favored Enemy. Sorry the sobriquet is so long, but it's traditional."
The most immediate "off" thing Simon seems to notice is that certain elements --business names, where certain road-lining plants are located, even an entire bridge-- aren't what they should be (or in the case of the bridge, there at all).
The one that doesn't click in his mind until a bit later is that a young man nearby, chatting with a newspaper salesman, looks remarkably like Simon's father. Except in his early 20s, tops.

"Hmm," the Professor says. "Are there amends you need to make with him? Critical turning point in his life that you might need to nudge in the right direction? There's usually a reason we end up places. Or the temporal guidance system is fitzing out again. That's possible."
As the wild-haired halfling ponders, two burly men grab Simon's dad as he leaves the newsstand and drag him into an alley.

They don't seem to notice Simon at all as they drag the man down a short passage and into a stockroom door.

The door they took Simon's father through leads to a nondescript room which contains a rather non-nondescript box, very similar to the Professor's, except it's solid black.
The two men force him into a chair, and the door of the box opens, revealing a man with an emaciated, tight-skinned head, completely hairless except for a goatee.

"Ah," the tall bald man says, "I thought this ruse might draw you into the open. You've thwarted my plans for the last time."