Since Piccolo knew Kami had special plans for Goku, it's likely that he believed Goku wasn't revived immediately because he was off receiving a yr's worth of special training for the Saiyans. Despite knowing nothing about Goku's training, Piccolo still considered him the strongest and their best chance at victory.
Based on Goku's importance against the Saiyans, his abrupt return would either signify that his training was complete, or they were going to have to fight the Saiyans sooner than expected. Goku asked Raditz when the other Saiyans would arrive, so I think it'd be reasonable for Piccolo to assume Goku needed this information so he'd be ready for them whenever they were close to arriving on Earth.
Can't really think of another answer for this one right now.