This answer to ** notes that it's hard to get current data on the orbits of Helios-1 and Helios-2 and I couldn't find any in JPL's Horizons.
They are shiny and in nice elliptical orbits between roughly 0.3 AU and 0.98 AU which means that from time to time they will be close to the Sun (and brightly reflective and quite bright in thermal IR for the NEO-seeking telescopes) or close to Earth and also bright in reflected light.
- When are close approaches of Helios 1 and 2 to the Earth?
- Have they ever turned up in astronomical sky surveys and been identified?
- Is there good orbital data or even state vectors available from while they were still active?
With nothing in Horizons I don't know where else to look.
From Helios 1
Engineer with Helios spacecraft before launch.
Screenshot from books.google.com New Eyes on the Sun: A Guide to Satellite Images and Amateur Observation by John Wilkinson
Figure 2.2. The trajectory of the Helios probes was unusual because they were design to make measurements in the medium between the inner planets as well as around the Sun. (Credit: NASA).