In Deathly Hallows Snape and Ron both independently find Harry and Hermione in the Forest of Dean.
Ron found them by using the Deluminator which guided him to them.
Snape found them after getting a tip-off from Phineas Nigellus.
Both of them were operating on Dumbledore's orders and using the means of finding Harry that Dumbledore had given them. Both of them were trying to find Harry in the forest, without being sure where exactly he was. Both of them were wandering around the forest at the same time that night. But, as I said, they arrived separately and each had their own indepedent means of knowing where Harry was.
We can be sure that Ron didn't know that Snape was there (since he was as clueless about who cast the doe Patronus as Harry is). But did Snape know that Ron was in the same forest seeking the same quarry?
He may have guessed that Ron was travelling with Harry and Hermione - I don't mean knowledge in that sense. I mean a knowledge that Ron was no longer in the tent with Harry and Hermione and was travelling around the forest on his own.
Did Snape know where Ron was, according to that definition? It would've been pretty awkward if they'd bumped into one another...