I think it was a combination of circumstances: LE asking for tips, a car driven erratically enough to draw attention, then W3 realizes it's a Porsche like the one LE mentioned (or not), W3 sees the driver is wearing a green safety vest, which would seem out of the ordinary to me, and make me look more closely at the driver. It's not a ton of info. W3 sees a Porsche driving erratically and the driver is wearing a green vest. Driver appears to the witness to be a clean cut African American man with short hair.
Assuming that's all the info called in, it would be after the burning car was found with the green vest inside that LE would know the tip was valuable. Who knows how many tips they received about Porsches that didn't pan out?