I presume you mean White's alleged sighting and not Smith's.
Whether the alleged sighting took place is a debatable point. The whole article should be treated with caution. My opinion is something happened on one of the nights of the murders but was exaggerated quite a bit in the Peoples Journal article. That's just a gut feeling by me and not based on any solid evidence.
The most common setting for the alleged sighting is Mitre Square (I favour one of the entrances into the Square myself). Some authors have suggested Castle Alley where others think it relates to Berner Street, unfortunately it's all guesswork.
Somehow this reminds me of the story about a police officer (Robert Sagar?) observing a man of Jewish appearance coming out of a court in Mitre Square (as reported in The Daily News of January 9, 1905, The Morning Leader of January 9, 1905, and The Seattle Daily Times of February 4, 1905). In two of the reports the officer is said to have discovered Eddowes' body immediately afterwards. The Jewish man is described as "well-known" in The Daily News and "well dressed" in The Seattle Daily Times. It's been suggested that this may be the same suspect mentioned in the memoirs of Sagar's colleague, Henry Cox?
If the incident refers to Berner Street, this reminds me of Edward Spooner meeting "Mr. Harris“ on Fairclough Street shortly after Stride's body had been discovered – but no PC was involved in that incident.