I too am surprised about him putting his stuff in the car, per the ABC news report. It must be his car, or was he putting stuff in someone else's car?
Did he not trust anyone so he kept his things locked in the trunk of a car when he was not where he was staying? Or was he planning another trip in his car?
Regarding his cell phone plan, perhaps he has a Greek SIM card. Still, his phone should have a personal ID number that his regular USA phone provider can trace. When I've travel internationally, I get a local SIM card. However, if I walked into a shop that was tracking traffic in and out of the store, my phone ID would show up, not my SIM card phone number. It's worth a shot for the family to contact Eric Albert's USA phone provider for location data.
If that photo is recent, maybe the friend in the photo has more information about Eric Albert's usual hiking route, and why he might have hiked 4 hours for a lunch appointment - rather than driven the car we see in CCTV footage. Was he Eric with some people, and Albert with others, or did everyone say Eric Albert?
I think it would be helpful to know more about his itinerary. We don't know when he planned to leave the Island, yet he is seen with extra luggage and a car on the morning of a day hike - same day? Is it confirmed that the CCTV of him with a car is from the morning of June 11?